PEPTIDE PURIFICATION BY MEANS OF HARD METAL ION AFFINITY CHROMATOGRAPHY FIELD OF THE INVENTION The present invention relates, inter alia, to the field of isolation and purification of peptides, notably polypeptides, such as recombinant proteins, by means of immobilized hard metal ion affinity chroma-tography. BACKGROUND OF THE INVENTION An important aspect of the production of recombinant (genetically engineered) peptides, including oligo- and polypeptides, notably proteins, intended for therapeutic use in humans or animals is purification of the peptides in question to a sufficiently high level of purity, such that the desired protein is essentially completely free of contamination with, in particular, (a) any extraneous proteins which may arise in the production process (typically a fermentation process or the like employing a selected or genetically modified strain of an appropriate microorganism) and (b) undesirable metal ions (notably heavy-metal ions) that may have been introduced in the course of the production process. Immobilized metal ion affinity chromatography (IMAC) is a versatile separation procedure that exploits differences in the affinities exhibited by many biopolymers for metal ions. The technique involves the chelation of a suitable metal ion onto a solid support matrix whose surface has previously been chemically modified with a polydentate ligand. The resulting immobilized metal ion chelating complex then has the potential to coordinate with one or more electron donor groups resident on the surface of the interacting protein (Sulkowski, E., Trends in Biotechnology, 3 (1985) 1-6; Porath, J., Carlsson, I., Ols-son, I. and Belfrage, G., Nature, 258 (1975) 598-599; Kagedal, L, in "Protein Purification" (Ed., J. C. Janson, and L. Ryden), VCH Publishers (1989) pp. 227-251; Zachariou, M. and Hearn, M. T. W., Biochemistry, 35 (1996) 202-211. Separation selectivity is then achieved on the basis of differences in the thermodynamic stabilities of the immobilized metal ion complexes with the various adsorbed proteins. Proteins whose adsorption complexes are the least stable will be eluted first, whilst proteins that form more stable complexes will be eluted later. The greater the difference in the equilibrium association constants, i.e. the larger the differences in the dissociation constants (Kg) of the respective protein/immobilized metal ion coordination complexes, the higher the resolution obtained. Consequently, the amino acid composition, surface distribution of particular amino acid residues, as well as the conformation of the protein all play important roles in determining the affinity of a protein for a particular IMAC system. As a result, proteins with very similar properties with respect to charge, molecular size and amino acid composition, but with differences in their tertiary structures, may be resolved. Most of the research interest into the use of IMAC over the past 20 years has revolved around the application of 1st row transition metal ions of borderline hardness (vide infra), such as Cu2*, Zn2* and Ni2+. These metal ions demonstrate intermediate metal ion stability constants, e.g. log(3 values be- tween 5 and 10, for both aromatic and aliphatic amines, as well as for carboxylate functional groups (Wong, J. W., Albright, R. L. and Wang. N. H. L, Separation and Purification Methods, 20 (1991) 49-57; Zachariou, M., Traverse, I.., Spiccia, L. and Hearn, M.T.W., Journal of Physical Chemistry, 100 (1996) 12680-12690). A number of unconstrained tridentate chelates that exhibit these binding properties with M2* ions can be chemically immobilized onto support materials. Despite their limitations with regard to the magnitude of the corresponding logp values and their resulting relatively low selectivity capabilities, unconstrained types of chelating compounds such as iminodiacetic acid (IDA) constitute the principal types of chelating ligand employed hitherto in such IMAC investigations [see, e.g., Kage-dal, L., in "Protein Purification" (Eds. J. C, Janson and L. Ryden), VCH Publishers (1989) pp 227-251]. Applications illustrative of the use of immobilized M2*-IDA-based IMAC systems include the purification of cc-amylases from germinated wheat using immobilized Cu2+-IDA [Zawistowska, U., Sangster, K., Zawistowski, J., Langstaff, J. and Friessen, A. D., Cereal Chemistry, 65(1988) 5413-5418]; and purification of human clotting factor VII [Weeransinghe, K. M., Scully, M. F. and Kadder, V. V., Biochimica Biophysica Acta, 839 (1985) 57-65] and of a-|-thiol proteinases [Otsuka, S. and Yamanaka, T. (Eds), "Metalloproteins -Chemical Properties and Biological Effects" in "Bioactive Molecules", Kodansha Ltd, Tokyo (1988), pp 18-45] from human plasma using immobilized Zn2+-IDA. An extension of the use of IDA-based IMAC procedures, viz. the purification of recombinant proteins using immobilized Ni2+-nitrilotriacetic acid (Ni2*-NTA) [Hochuli, E., Bannwarth, W., Dobeli, H. and Stuber, D.t Bio/Technology, 6 (1988) 1321-1324] (NTA being a structural homologue of IDA), relies on the incorporation at the gene level of a polynucleotide sequence corresponding to a poly-histidine peptide, typically hexa-His, which confers on the protein a higher affinity for binding to immobilized Ni2*-NTA chelating complex, thus enabling the protein to be selectively retained on this IMAC sorbent. In the present application, the terms "sorbent" and "adsorbent" are used primarily to denote a functionalized polymer substrate (polymer substrate with ligand immobilized thereto) with coordinatively bound metal ion(s), although these terms are also occasionally employed to denote a functionalised polymer substrate without metal ion(s) bound thereto. As will be noted from the above description of applications of IDA- and NTA-based IMAC systems, an alternative means of altering protein binding selectivity with IMAC systems is through variation in the structure of the chelating ligate. In recent years however, only a handful of new IMAC chelating ligates have been introduced. These include systems based on the bidentate chelators aminohydroxamic acid (AHM) and 8-hydroxyquinoline (8-HQ) [Zachariou, M., Traverso, I., Spiccia, L. and Hearn, M.T.W., Journal of Physical Chemistry, 100 (1996) 12680-12690]; carboxymethylaspartic acid (CM-ASP) which has a higher affinity for Ca2+ than IDA [Porath, J., Trends in Analytical Chemistry, 7 (1988), 254-256; Mantovaara, T., Pertofz, H. and Porath, J., Biotechnology Applied Biochemistry, 11 (1989), 564-569]; orffto-phosphoserine (OPS), which is able to chelate "hard" metal ions such as Fe3+, Al3*, Ca2* and Yb3* due to the participation of the phosphate group [Zachariou, M., Traverso, I. and Hearn, M. T. W., Journal of Chromatography, 646 (1993), 107-115]; and other tridentate ligates, such as (2-pyridylmethyl)aminoacetate (CPMA), dipicolylamine (DPA) and c/s- or frans-carboxymethyl-proline [Chaouk, H., Middleton S., Jackson W.R. and Hearn, M.T.W.. International Journal of BioChromatogra- phy, 2 (1997) 153-190; Chaouk, H. and Heam, M.T.W., Journal of Biochemical and Biophysical Research Methods, 39 (1999) 161-177], tetradentate ligands, such as nitrilotriacetic acid (NTA) [Hochuli, E., Bannwarth, W., Dobeli, H., Gentz, R. and Stuber, D. Bio/Technology, 6 (1988) 1321-1325], which have higher affinities for M2* ions than IDA due to their quadridentate nature, exhibit lower protein binding association constants due to the loss of one coordination site compared to the IDA-type tridentate ligates; and pentadentate ligands, such as tetraethylenepentamine (TEPA) [Hidaka Y., Park, H. and Inouye, M., FEBS Letters, 400 (1997) 238-242] or N,N,N''-tris(carboxymethyl)ethylene-diamine (TED) [Porath, J., Protein Expression & Purification, 3 (1992) 263-281], which coordinate metal ions via five donor atoms (i.e. two nitrogen atoms of primary amine groups and three nitrogen atoms of secondary amine groups in the case of TEPA, and two nitrogen atoms of secondary amine groups and three oxygen atoms from the three carboxylic groups in the case of TED). Significant leakage of metal ions has been observed with immobilized metal ion iminodiacetic acid chelate (im-Mn+-IDA) systems when using relatively mild elution conditions in the chromatographic process [Oswald, T., Hornbostel, G., Rinas, U. and Anspach, F.B., Biotechnology Applied Biochemistry, 25 (1997) 109-115; Kagedal, L. in Protein Purification (eds. J.C. Janson and L Ryden) VCH Publishers, New York (1989), pp 227-251]. Thus, in addition to the issue of selectivity modulation, an additional motivation for the development of new classes of chelating ligates has been a need for achieving significant increases in the metal ion stability constants compared to the IDA-based or NTA-based systems which have hitherto been employed [Zachariou, M., Traverse, I., Spiccia, L. and Heam, M.T.W., Analytical Chemistry, 69(1996)813-822]. WO 03/042249 relates, inter alia, to classes of functionalized polymer substrates containing a functionality comprising one or more cyclic, metal ion coordinating ligand groups having at least 3 metal ion coordinating donor atoms chosen independently among N, O and S. When employed as a matrix for one or more metal ions that form(s) coordination bonds to these donor atoms whilst retaining vacant coordination sites, these functionalized polymer substrates were found to exhibit remarkably high strength and/or selectivity of binding towards fusion proteins in the form of proteins or polypeptides "tagged" with an additional oli-gopeptide sequence (''lag") incorporating one or more appropriately positioned amino acid residues capable of forming a coordination bond to the vacant coordination site(s) of the metal ion or ions in question. The generally significantly greater strength and/or selectivity of binding of the fusion protein to such a matrix compared with that of the binding of extraneous proteins then facilitated separation and isolation of the fusion protein from a mixture containing the fusion protein together with one or more extraneous proteins. Preferred functionalized (metal ion coordinating) polymer substrates disclosed in WO 03/042249 employ functionalities in which the metal ion coordinating donor atoms in each cyclic, metal ion coordinating group consist of three nitrogen donor atoms in the ring, and they are particularly well suited for use as a matrix for certain metal ions of borderline properties with respect to "hardness" or "softness" (vide infra), such as Cu2* or Ni2*. Functionalized polymer substrate systems well suited as a matrix for "hard" metal ions (such as Ca2*, Mg2*, Mn2* and Fe3*) or metal ions at the "hard" end of the scale with respect to "borderline" hardness (such as Zn2*) are less well represented in WO 03/042249. BRIEF DESCRIPTION OF THE INVENTION One objective of the present invention was to provide novel IMAC systems based on "hard" metal ions (such as Ca2*, Mg2+ and Fe3+) and metal ions at the "hard" end of the scale with respect to "borderline" hardness (such as Zn2*) that function through interactions with hard donor atoms [especially oxygen atoms in carboxylate groups (as in Asp or Glu amino acid residues) and/or phosphate groups] present in biomolecules. An important feature of these novel IMAC-based systems is that they can concurrently achieve a mixed modality of interaction with their target molecules that is based on a combination of coordination (electron donor/electron acceptor) and electrostatic (ion-exchange) processes. As a consequence, these novel IMAC-based systems can function with selectivity mediated through mixed modes of interaction that are unique and thus offer the opportunity for a new capability in protein purification. One aspect of the present invention thus relates to a polymer substrate functionalized with a functionality comprising at least one cyclic, metal ion coordinating ligand group which comprises at least 3 nitrogen donor atoms in the ring of said cyclic group, at least one of said nitrogen atoms having an optionally substituted carboxy(lower alkyl) group or an optionally substituted phosphono(lower alky!) group covalently attached thereto. A second aspect of the invention relates to a functionalized polymer substrate of the latter type, further comprising a metal ion coordinated to at least one of the cyclic ligand groups in the functionality. Other aspects of the invention include methods for preparing such functionalized polymer substrates. Further important aspects of the present invention relate to: oligopeptides that are well suited for incorporation as "tags" in fusion proteins in the context of the present invention; fusion proteins of the type in question, comprising a protein of interest fused at its amino terminus or carboxy terminus or both, or alternatively at a location within the internal amino acid sequence of the protein of interest, to at least one such oligopeptide; polynucleotide constructs, e.g. vectors, encoding such fusion proteins; host cells that comprise such a polynucleotide construct; a method for producing a fusion protein of the type in question, wherein a host cell of the latter type is cultivated in a growth medium under conditions whereby the fusion protein is expressed, and whereby the fusion protein is recovered from the medium; and a method for purifying a protein of interest, wherein a wild-type protein or a protein sample containing such a fusion protein (comprising the protein of interest) as well as other proteins (extraneous proteins) is contacted with a functionalized polymer substrate according to the invention or a metal ion-containing functionalized polymer substrate according to the invention. DETAILED DESCRIPTION OF THE INVENTION As already indicated above, a first aspect of the invention relates to a polymer substrate functionalized with a functionality comprising at least one cyclic, metal ion coordinating ligand group which comprises at least 3 nitrogen donor atoms in the ring of the cyclic group, at least one of the nitrogen atoms having an optionally substituted carboxy(lower alkyl) or optionally substituted phosphono(lower alkyl) group covalently attached thereto. Useful polymer substrates in the context of the invention include both water-soluble polymers and substantially water-insoluble polymers, and may be selected from a very wide range of polymeric materials. Examples hereof are the following: Polvsaccharides and derivatives thereof, including agaroses, dextrans, celluloses, hemicelluloses, starches, xylans and the like, and derivatives of these polysaccharides. Suitable polysaccharide derivatives will, in general, include derivatives in which some proportion of the hydroxy groups of the polysaccharide in question is derivatized to form ethers (e.g. lower alkyl ethers, such as methyl ethers) or esters (e.g. lower carboxylic acid esters, such as acetate, propionate and the like), as well as materials in which the starting polysaccharide or a derivative thereof has been cross-linked by treatment with an appropriate cross-linking reagent. Generally speaking, functionalized polymer substrates of the invention based on substantially water-insoluble polymers are, for example, well suited for packing into chromatography columns, for direct introduction into a medium (batchwise use) and the like, and polysaccharides that are particularly well suited for this type of application in the context of the invention include agaroses, dextrans and derivatives thereof, a variety of suitable types of which are readily commercially available. Thus, for example, a variety of agarose products are produced by Amersham Pharmacia Biotech, Uppsala, Sweden, and marketed under the name Sepharose™; available grades include Sepharose™ 28,4B and 6B. Cross-linked derivatives of these various grades of agarose (prepared by cross linking of Sepharose1" with 2,3-dibromopropanol) are also available from the same company, and are marketed as Sepharose™ CL-2B, CL-4B and CL-6B, Sepharose™ 4 and 6 Fast Flow, Sepharose™ 6MB, and Superose™ 6 and 12, respectively. A number of dextran-based or dextran-agarose composite materials suitable for use in the context of the present invention are also available from Amersham Pharmacia Biotech under the names Sephadex™, Superdex™ (e.g. Superdex™ 30, 75 and 200) and Sephacryl™. Products in the Sephadex™ range are prepared by cross-linking dextran with epichlorohydrin and are available in the following grades: Sephadex™ G-10, G-15, G-25, G-50, G-75, G-100, G-150 and G-200, the degree of cross-linking decreasing with increasing G number. Products in the Sephacryl™ range are prepared by cross-linking allyl-dextran with A/./V-methylene-bisacrylamide, and include Sephacryl™ S-100, S-200, S-300, S-400, S-500 and S-1000; the latter six products differ with respect to their range of pore size and particle size distribution. Products in the Superdex™ range are prepared by cross-linking allyl-dextran with agarose derivatives of various compositions. Polyalkvlene glvcols and derivatives thereof, including, in particular, polyethylene glycols (PEG), i.e. condensation polymers of ethylene glycol having the general formula HOCH2(CH2OCH2)nCH2OH or H(OCH2CH2)nOH and typically having average molecular weights in the range from 200 to 6000. A number of PEG''s (including PEG''S of average molecular weight 400, 600, 1500, 4000 and 6000, respectively) are available under various names (e.g. Macrogol™, PEG™, Carbowax™, Nycoline™, Plu-racol E™, Poly-G™, Polyglycol E™, Solbase™) from a variety of commercial sources. PEG''S are generally soluble in or miscible with water, as well as in ethanol and a number of other organic solvents, including aromatic hydrocarbons. The analogous polypropylene glycols [having the general formula H(OC3H6)nOH], the lower molecular weight members of which are soluble in water, are also of relevance in the context of the invention. Relevant derivatives of such polyalkylene glycols include partially etherified derivatives, e.g. derivatives in which one of the terminal hydroxy groups has been converted to a lower alkyl ether group, such as a methyl ether group. Such polymers can readily be immobilized to support materials, thereby producing substrates that can subsequently be activated and then functionalized or derivatized with macrocyclic metal ion binding chelatihg ligands by procedures according to the present invention. Polyvinyl polymers, including polyvinyl alcohols - i.e. hydroxylic polymers normally produced by hydrolysis ("alcoholysis") of various molecular weight fractions of polyvinyl acetate, typically by base or acid hydrolysis - and derivatives thereof. The degree of "alcoholysis" may be varied by either allowing the hydrolysis of acetate ester groups in polyvinyl acetate to proceed to substantial completion, or by stopping it at a desired degree of alcoholysis. Polyvinyl alcohols are normally commercially available in four molecular weight ranges, viz. ca. 250,000-300,000 (termed super-high viscosity), ca. 170,000-ca. 220,000 (termed high-viscosity), ca. 120,000-150,000 (termed medium-viscosity) and ca. 25,000-ca. 35,000 (termed low-viscosity). In general, the lower the molecular weight of polyvinyl alcohols, the higher is their water sensitivity or ease of water solubility; however, the degree of alcoholysis also plays a role with regard to the water-solubility and other properties of polyvinyl alcohols. Polyvinyl alcohols within all of the above-outlined categories are or relevance in the context of the present invention, as are, for example, ether derivatives thereof, such as methyl ether derivatives. Other polyvinyl polymer materials of interest include materials such as the Toyopearl™ HW range of porous, semi-rigid spherical gel particles designed for medium- and low-pressure liquid chromatogra-phy. Such materials, after activation and functionalization/derivatization, provide another option for the preparation of IMAC sorbents of relevance in the context of the invention. Toyopearl™ HW gels (obtainable from Tosoh Corp, Yamaguchi, Japan, and other suppliers) are synthesized from hydrophilic vinyl polymer containing exclusively C, H and O atoms. Available grades (differing with respect to particle and pore sizes) include Toyopearl™ HW-40, HW-40C, HW-40F, HW-40S, HW-50, HW-50F, HW-50S, HW-55, HW-55F, HW-55S, HW-65F, HW-65S and HW-75F. Polvacrvlamides and derivatives thereof, including composite materials based on polyacrylamide and agarose, such as Ultrogel1'''' AcA gels (composite polyacrylamide-agarose gel in bead form, available from, e.g., Amersham Pharmacia Biotech). The Ultrogel™ AcA gel range includes AcA 22, AcA 34, AcA 44 and AcA 54, where the number refers to the percentage of acrylamide and agarose, i.e., AcA 22 contains 2% acrylamide and 2% agarose. Activation of hydroxylic groups of these support materials provides an avenue to the preparation of IMAC sorbents. Surface-modified silicas, including glycidylpropoxy-modified porous silica, such as LiChroSpher™ Diol (E. Merck, Darmstadt, Germany), Toyosoda™ TSKSW3000 (Tosoh Corp., Yamaguchi, Japan); amino-propyl-modified silica, prepared by reaction (in the presence of a suitable catalyst) of aminopropyldi-ethoxysilane with silicas of appropriate pore size and appropriate average diameter; and mercapto-propylsilicas, prepared by reaction (in the presence of a suitable catalystj''of mercaptopropyldiethoxysi-lane with silicas of appropriate pore sizes and appropriate average diameters. Alternatively, dextran modified or butadiene-vinyl copolymer modified silicas of appropriate pore sizes and appropriate average diameters can be employed as the chromatographic support materials. "Naked" porous silicas suitable for such derivatization and subsequent modification to generate the respective novel IMAC sorbents can readily be obtained from a variety of suppliers, including E. Merck, (Darmstadt, Germany), Tosoh Corporation, Yamaguchi, Japan), Eka-Nobel AB (Goteborg, Sweden) and Grace Davi-son GmbH (Worms, Germany). Surface-modified metal oxides, including glycidylpropoxy-modified porous zirconias, titanias or aluminas, as well as modifications/variants thereof based on the respective metal oxide "doped" with a second metal oxide; amino-propyl-modified zirconia, titania or alumina, prepared by reaction (in the presence of a suitable catalyst) of aminopropyldiethoxysilane with the zirconia, titania or alumina of appropriate pore size and appropriate average diameter; and mercaptopropyl-modified zirconia, titania or alumina, prepared by reaction (in the presence of a suitable catalyst) of mercaptopropyldiethoxysi-lane with the zirconia, titania or alumina of appropriate pore size and appropriate average diameter. Alternatively, dextran modified or butadiene-vinyl copolymer modified zirconia, titania or alumina of appropriate pore sizes and average diameters can be employed as the chromatographic support materials. "Naked" porous zirconia, titania or alumina suitable for such derivatization and subsequent modification to generate the respective novel IMAC sorbents can readily be obtained from a variety of suppliers, including YMC Co. Ltd. (Kyoto, Japan), Grace GmbH (Worms, Germany) and BioSepra Corp. (Paris, France). Well suited polymer substrates in the context of the invention include agaroses, dextrans and derivatives thereof, e.g. materials selected among those outlined above. In one aspect of the invention, the cyclic, metal ion coordinating ligand group in a functionalized polymer substrate according to the invention is derived from a heterocycle chosen among: triazacycloal-kanes and -cycloalkenes; and tetraazacycloalkanes and -cycloalkenes. Among such functionalized polymer substrates, particularly well suited cyclic, metal ion coordinating ligand groups include groups derived from a heterocycle chosen among the following: 1,4,7-triazacyclononane; 1,4,7-triazacyclodecane; 1,4,8-triazacycloundecane; 1,5,9-triazacyclododecane; 1,4,7,10-tetraazacyclododecane; 1,4,7,10-tetraazacyclotridecane; 1,4,7,11-tetraazacyclotetradecane; 1,4,8,11- tetraazacyclotetradecane; 1,4,8,12-tetraazacyclopentadecane; and 1,5,9,13-tetraazacyclohexadecane. In relation to this latter aspect of the invention, a hydrogen atom in one or more ring -CH2- groups or -CH= groups (and/or, in some instances, in a ring -NH- group) in one of the above cyclic, metal ion coordinating ligand groups of the tri- or tetraazacycloalkane or -cycloalkene type may optionally be substituted with a substituent selected among optionally substituted lower alkyl groups and optionally substituted aryl groups; further examples of optional substituents appropriate for substituting a hydrogen atom in, in particular, a ring -CH2- or -CH= group in a metal ion coordinating ligand groups of the tri- or tetraazacycloalkane or -cycloalkene type include optionally substituted lower alkoxy groups. The optional substituent(s) on the lower alkyl group, lower alkoxy group or aryl group in question may optionally comprise one or more metal ion coordinating donor atoms, such as one or more O or S donor atoms. The term "lower alkyl" as employed in the context of the present invention in intended to designate any linear (straight-chain), branched or cyclic alkyl group having from 1 to 6 carbon atoms. Examples of linear alkyl groups are methyl, ethyl, propyl, butyl, pentyl and hexyl; examples of branched alkyl groups are isopropyl, iso-butyl, sec-butyl, tert-butyl, isopentyl and isohexyl; examples of cyclic alkyl groups are cyclopropyl. cyclobutyl, cyclopentyl and cyclohexyl. In general, linear or branched lower alkyl groups having from 1 to 3 carbon atoms (i.e. methyl, ethyl, propyl and isopropyl) will be well suited in the context of the invention. Suitable optional substituents on lower alkyl groups in the context of the invention include halogen, hydroxy, lower alkoxy and optionally substituted aryl. The term "lower alkoxy" as employed in the context of the present invention in intended to designate any linear, branched or cyclic alkoxy group having from 1 to 6 carbon atoms. Examples of linear alkoxy groups are methoxy, ethoxy, propoxy, butoxy, pentoxy and hexoxy; examples of branched alkoxy groups are isopropoxy, sec-butoxy, tert-butoxy, isopentoxy and isohexoxy; examples of cyclic alkoxy groups are cyclopropyloxy, cyclobutyloxy, cyclopentyloxy and cyclohexyloxy. In general, linear or branched lower alkoxy groups having from 1 to 3 carbon atoms (i.e. methoxy, ethoxy, propoxy and isopropoxy) will be well suited in the context of the invention. In the present context, the term "aryl" is intended to designate any aromatic group and includes both carbocyclic and heterocyclic aromatic groups. Examples thereof are phenyl, naphthyl, pyridyl, tetrazolyl, thiazolyl, imidazolyl, indolyl, quinolinyl, pyrimidinyl, thiadiazolyl, pyrazolyl, oxazolyl, isoxazolyl, thienyl, furanyl or oxadiazolyl. Suitable optional substituents on aryl groups in the context of the invention include halogen, amino, hydroxy, lower alkyl and lower alkoxy. The term "halogen" designates Cl, F, Br or I. With regard to the optionally substituted carboxy(lower alkyl) or optionally substituted phos-phono(lower alkyl) group covalently attached to at least one of the ring N atoms of the cyclic, metal ion coordinating ligand group, carboxymethyl (-CH2COOH) and phosphonomethyl [-CH2PO(OH)2], respectively, have proved to be very suitable. With cyclic, metal ion coordinating ligand groups of the triaza-cycloalkane or -cycloalkene type, mentioned above, it appears to be advantageous that at least two (i.e. two or three) of the three ring N atoms have an optionally substituted carboxy(lower alkyl) or optionally substituted phosphono(lower alkyl) group (e.g. a carboxymethyl or phosphonomethyl group) covalently attached thereto. In the case of metal ion coordinating groups of the tetraazacycloalkane or -cycloalkene type, mentioned above, it appears to be advantageous that at least two (i.e. two, three or four) of the four ring N atoms have an optionally substituted carboxy(lower alkyl) or optionally substituted phosphono(lower alkyl) group (e.g. a carboxymethyl or phosphonomethyl group) covalently attached thereto. Suitable optional substituents on the lower alkyl moiety in such optionally substituted carboxy(lower alkyl) or optionally substituted phosphono(lower alkyl) groups (e.g. optional substituents on the -CH2-moiety of a carboxymethyl or phosphonomethyl group) include optionally substituted aryl, i.e. aryl (e.g. phenyl) and substituted aryl [e.g. (lower alkyl)phenyl, such as methylphenyl, ethylphenyl, propylphenyl, isopropylphenyl, cyclopropylphenyl, cyclobutylphenyl, cyclopentylphenyl or cyclohexylphenyl, where the lower alkyl group on the phenyl ring may be in any position (i.e. 2-, 3- or 4- position) relative to the carbon atom bearing the carboxy group or phosphono group. It is generally advantageous that the functionality in a functionalized polymer substrate according to the present invention is covalently attached to the polymer substrate by means of a linker or spacer group X, the group X being attached to a ring nitrogen atom of the cyclic, metal ion coordinating ligand group. The linker or spacer group X may be any suitable type of linker or spacer, but will typically be one which may be derived from a bifunctional organic compound (e.g. an organic compound having at least two reactive functional groups selected from groups such as carboxyl, thiol, aminopropyl, halogen and epoxy) by reaction with, on the one hand, an appropriate reactive functionality on the polymer substrate and, on the other hand, the appropriate reactive functionality - in this case a substituted amino group (-NH-) in the ring - of a cyclic, metal ion coordinating ligand group. A type of linker or spacer group X which is generally very useful in the context of the present invention is one which can be derived from epichlorohydrin by reaction of the halogen end thereof with, e.g., an hydroxy group on the surface of the polymer substrate in question and then reaction of the epoxy group thereof with a substituted amino group in a cyclic ligand group. In particularly useful embodiments of functionalized polymer substrates of the invention the linker or spacer group X is a group derivable from epichlorohydrin by reaction thereof with the polymer substrate in the form of an agarose or agarose derivative, and subsequent reaction of the resulting product with a ring -NH- group of the cyclic, metal ion coordinating ligand group which becomes bound to X. As is apparent from the disclosure herein, an important feature of the materials (functionalized polymer substrates) employed according to the invention to isolate and purify a desired protein (protein of interest) is the presence, in the material, of a metal ion which itself is bound coordinatively to a cyclic ligand group in the functionality, and which in turn is capable of binding coordinatively, and suitably selectively, to donor atoms in the amino acid residues of the oligopeptide "tag" part of a fusion protein in which the oligopeptide "tag" is attached to the amino acid sequence of the protein of interest. A further aspect of the invention thus relates to a functionalized polymer substrate as described above, in which at least one of the cyclic ligand groups in the functionality has a metal ion coordinated thereto. Functionalized polymer substrates disclosed and described herein are particularly well suited to coordination of certain divalent (2-t-charged) or trivalent (3+-charged) metal ions, notably metal ions chosen among Ca2+, Mg2+, Zn2* and Fe3*. As illustrated by working examples provided herein (vide infra), Ca2* is a versatile metal ion in this connection. As already indicated briefly, a further aspect of the invention relates to a process for preparing a functionalized polymer substrate according to the invention, the process comprising the steps of: selecting a polymer substrate having a reactive functional group capable of undergoing a first reaction with a first functional group of a bifunctional reagent having a first and a second functional group; the first reaction in question resulting in covalent bond formation between the polymer substrate and the bifunctional reagent; the second functional group of the resulting covalently bound reagent being subsequently capable of undergoing a second reaction with a reactive ring -NH- group present in a species comprising at least one cyclic, metal ion coordinating ligand group which comprises at least 3 nitrogen donor atoms in the ring of the cyclic group, at least one of the nitrogen atoms in question having an optionally substituted carboxy(lower alkyl) or phosphono(lower alkyl) group covalently attached thereto; and the second reaction resulting in covalent bond formation between the species in question and the covalently bound reagent; reacting the polymer substrate with the bifunctional reagent; and reacting the resulting covalently bound reagent with the species in question. In relation to the latter process according to the invention, the polymer substrate employed, and the cyclic, metal ion coordinating ligand group in the reactive species employed in the process, may be chosen among those already discussed above in connection with functionalized polymer substrates according to the invention. The bifunctional reagent employed will typically be a bifunctional organic compound, e.g. an organic compound having at least two reactive functional groups chosen among groups such as carboxyl. thiol, aminopropyl, halogen and epoxy. Epichlorohydrin is particularly useful as a bifunctional reagent for a number of types of polymer substrate, including polysaccharides and derivatives thereof having surface hydroxyl groups. As will be apparent from the discussion above in relation to functionalized polymer substrates according to the invention, the reactive species containing the cyclic, metal ion coordinating ligand group will suitably be one which gives rise to a functionality (in the resulting functionalized polymer substrate product) of one of the types described above. Thus, appropriate reactive species for use in the process of the invention will then include species containing one or more cyclic, metal ion coordinating ligand groups having a reactive ring -NH- group and being derived from heterocycles chosen among: triazacycloalkanes and -cycloalkenes or among tetraazacycloalkanes and -cycloalkenes, e.g. species containing one or more cyclic, metal ion coordinating ligand groups having a reactive ring -NH- group and being derived from heterocycles chosen among: 1,4,7-triazacyclononane; 1,4,7-triazacyclodecane; 1,4,8-triazacycloundecane; 1,5,9-triazacyclododecane; 1,4,7,10-tetraazacyclododecane; 1,4,7,10-tetraazacyclotridecane; 1,4,7,11-tetraazacyclotetradecane; 1,4,8,11- tetraazacyclotetradecane; 1,4,8,12-tetraazacyclopentadecane; and 1,5,9,13-tetraazacyclohexadecane. The considerations above (in the context of functionalized polymer substrates of the invention) with regard to the optionally substituted carboxy(lower alkyl) or phosphono(lower alkyl) group covalently attached to at least one of the ring N atoms of a cyclic, metal ion coordinating ligand group likewise apply to the above-described process of the invention. Thus, carboxymethyl (-CH2COOH) and phos-phonomethyl [-CH2PO(OH)2] are very suitable as optionally substituted carboxy(lower alkyl) groups and phosphono(lower alkyl) groups, respectively. As will also be apparent from the foregoing discussion, agaroses and agarose derivatives (such as Sepharose™ products as described above) are well suited as polymer substrates in the context of the above-described process according to the invention. A well-suited bifunctional reagent will then be epichlorohydrin, and it may be advantageous in this connection to further incorporate a reducing agent, such as sodium borohydride, in the reaction mixture when reacting the polymer substrate with epichlorhydrin. The scope of the present invention further encompasses functionalized polymer substrates obtained or obtainable by a process as described above for preparing a functionalized polymer substrate. In addition, the scope of the present invention also encompasses a process for preparing a functionalized polymer substrate which is in accordance with the invention, and which further comprises a metal ion coordinated to at least one of the cyclic, metal ion coordinating groups therein, the process comprising contacting a functionalized polymer substrate according to the invention with an aqueous solution of an inorganic salt [e.g. a nitrate, halide (fluoride, chloride, bromide or iodide), sulfate, perchlo-rate, tetrafluoroborate, hexafluorophosphate or phosphate salt] or organic salt [e.g. a carboxylate (such- as formate, acetate, propanoate or benzoate), tetraphenylborate or sulphonate salt] of the metal ion in question (e.g. one of the metal ions already mentioned above). A metal ion-containing functionalized polymer substrate obtained or obtainable by such a process is also within the scope of the present invention. Selection and preparation of metal ion coordinating (chelating) ligands having strong affinity for hard metal ions (and metal ions of borderline hardness): An important and valuable applicatior of functionalized polymer substrates as defined in the context of the present invention is the use of a metal ion containing embodiment thereof in the purification of a protein, the protein in question being in the form of a fusion protein wherein the protein of interest is fused at its amino or carboxy terminus to an oligopeptide "tag", such as an Asp-containing oligopeptide according to the invention. In addition, the simultaneous fusion of two molecules of a protein or polypeptide of interest, or alternatively two different proteins or polypeptides of interest attached at their amino- or carboxy- terminus, respectively, to an oligopeptide "tag", such as an Asp-containing oligopeptide according to the invention, generates a new fusion protein structure whereby the oligopeptide "tag" is located at an encto-position (i.e. an internal position) linking the two molecules of the protein(s) or polypeptide(s) of interest. Not all chelating ligands fulfil the requirements to be a suitable ligand for Immobilised Metal Affinity Chromatography (IMAC). The chelating ligands serve two aims: (a) they fix the metal ion to a solid support and (b) they modulate the metal affinity binding and thus the strength and affinity specificity of the adsorption centre.1 Ideally, the chelating ligand should form stable complexes with the metal ions so that no metal ions are released into the solvent phase or transferred to the biomolecules during adsorption and desorption of these molecules. At the same time it should also leave at least one, and preferably two or more coordination sites of the metal ion available for protein binding. R (Table Remove) Scheme 1: Ligand immobilised to a matrix (polymer substrate) Six ligands, based on the macrocycles cyclen and tacn, and containing carboxymethyl or phosphono-methyl pendant arms, have been synthesised on the basis of published methods2"4. These ligands are 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DO3A), 1,7,-bis(phosphonomethyl)-1,4,7,10-tetraazacyclododecane (DO2P), 1,4,7-tris(phosphonomethyl)-1,4,7,10-tetraazacyclodo-decane (DO3P), 1-(carboxymethyl)-1,4,7-triazacyclononane (T1A), 1,4-bis(carboxymethyl)-1,4,7-triazacyclononane (T2A) and 1,4-bis(phosphonomethyl)-1,4,7-triazacyclononane (T2P). As shown in Scheme 1, a secondary amine group enables attachment to an activated gel (polymer substrate), generating the IMAC support (functionalized polymer substrate). The stability constants of the corresponding Ca2*- complexes, reported by Burai et al 5 for DO2P (log Kca(ii)L=12.8) and by Chang53 for DO3A (log Kca