Powder comprising polymer-coated inorganic particles The invention relates to a powder based on polymer-coated fillers which has advantages in temns of the stability of the production process, and density, to the use of the said powder in shaping processes, and also to mouldings produced by a layer-by-layer process by which regions of a powder layer are selectively melted, with use of the said powder. After cooling and solidification of the regions previously melted layer-by-layer, the moulding can be removed from the powder bed. The mouldings according to the invention moreover exhibit less susceptibility to warpage than conventional mouldings. A task frequently encountered in very recent times is the rapid provision of prototypes. Particularly suitable processes are those which are based on pulverulent materials and in which the desired structures are produced layer-by-layer through selective melting and solidification. Supportive structures for overhangs and undercuts can be omitted here, because the powder bed surrounding the molten regions provides sufficient support. Nor is there any need for the subsequent operation of removing supports. The processes are also suitable for producing short runs. The selectivity of the layer-by-layer process here can be provided by way of example by applying susceptors, absorbers, or inhibitors, or by masks, or by way of focussed introduction of energy, for example through a laser beam, or by way of glass fibres. The energy is introduced by way of electromagnetic radiation. A process which has particularly good suitability for the purpose of rapid prototyping is selective laser sintering. In this process, plastics powders are briefly irradiated selectively in a chamber by a laser beam, and the powder particles which encounter the laser beam therefore melt. The molten particles coalesce and rapidly resolidify to give a solid mass. This process can provide simple and rapid production of three-dimensional products by repeated irradiation of a succession of freshly applied layers. The laser sintering (rapid prototyping) process for producing mouldings from pulverulent polymers is described in detail in the Patents US 6 136 948 and WO 96/06881. A wide variety of polymers and copolymers is claimed for the said application, examples being polyacetate, polypropylene, polyethylene, ionomers and polyamide. other processes with good suitability are the SIB processes described in WO 01/38061, and a process described in EP 1 015 214. Both processes operate with large-surface-area infrared heating for melting of the powder. The selectivity of the melting process is achieved in the first case by applying an inhibitor, and in the second process it is achieved by a mask. DE 103 11 438 describes another process. In this, the energy required for the fusion process is introduced through a microwave generator, and the selectivity is achieved by applying a usceptor. Other suitable processes are those operating with an absorber which is either present in the powder or is applied by ink-jet processes, as described in DE 10 2004 012 682.8, DE 10 2004 012 683.6 and DE 10 2004 020 452.7. The rapid prototyping or rapid manufacturing processes mentioned (RP or RM processes) can use pulverulent substrates, in particular polymers, preferably selected from polyesters, polyvinyl chloride, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, poly(N-methylmethacrylimides) (PMMI), polymethyl methacrylate (PMMA), ionomer, polyamide, or a mixture thereof. WO 95/11006 describes a polymer powder which is suitable for the laser sintering process and which, when melting behaviour is determined by differential scanning calorimetry with a scanning rate of fomn 10 to 20°C/min, exhibits no overlap of the melting and recrystallization peak, has a degree of crystallinity of from 10 to 90%, likewise determined by DSC, has a number-average molecular weight Mn of from 30 000 to 500 000, and has a Mw/Mn quotient in the range from 1 to 5. DE 197 47 309 describes the use of a nylon-12 powder which has increased melting point and increased enthalpy of fusion and which is obtained by reprecipitation of a polyamide previously produced through ring-opening and subsequent polycondensation of laurolactam. This is a nylon-12. DE 10 2004 003 485 describes the use of particles with at least one cavity for use in processes that build layers. All of the particles here comprise at least one cavity, and the particles comprising the cavity are melted by introduction of electromagnetic energy. Tlie powder particles described have a thin surface layer. DE 102 27 224 describes a granulated material which is intended for 3D binder printing and which is composed of particles provided with a surface layer comprising a non-polar external area. The surface layer of the powder particles described is, however, thin. In the prior art, the powders described above are sometimes mixed with other particles for reinforcement, e.g. metal particles, glass particles or Ti02 particles. However, a disadvantage here is that the handling of powder mixtures of this type often leads to demixing phenomena, and the mechanical properties that the reinforcing material is intended to achieve therefore sometimes vary. The regions where the proportion of filler is too high become very brittle and therefore unusable, and the regions comprising too little filler are softer than intended. The demixing derives from the different density of the polymer particles and of the filler, and tends to be apparent to some extent during any transport of the powder mixture and during its handling. In particular if the handling of the powder is automated in the rapid manufacturing process, it is difficult to control deviations in the properties of the components produced. WO 2007/051691 describes processes for producing ultra-fine powders based on polyamides, by precipitating polyamides in the presence of inorganic particles, where a suspension is used with inorganic particles suspended in the alcoholic medium, where the dso median size of the inorganic particles is in the range from 0.001 to 0.8 pm. Fine polyamide powders were obtained here, and, because of their small size, the inorganic particles have unifomri distribution in the composite particles here. The said process was aimed at achieving colouring of the powder and of the moulding fomied therefrom. The said measure does not alter the mechanical properties of the moulding. It was an object of the present invention to eliminate the problem of the demixing phenomenon and to achieve an improvement in the consistency of mechanical properties in the moulding, preferably strengthening of the moulding, and flame retardancy and/or an improvement in thermal conductivity, where these are intended to be achieved with the reinforcing material. The technical object was achieved through a powder for use in a layer-by-layer process for producing mouldings by selectively melting regions of the respective powder layer through introduction of electromagnetic energy, comprising composite particles which are produced from core particles completely or partially coated with a precipitated polymer, where the core particles are inorganic core particles, with the exception of titanium dioxide, and where the dso median diameter of the inorganic core particles is from 1 to 70 pm. The dso median diameter of the core particles in all three spatial directions here is from 1 to 70 pm. The dimensions in each spatial direction here can be different. It is preferable that the dso median diameter of the core particles in all three spatial directions independently of one another is from 1 to 70 pm. The data for the diameters of the core particles is based here on the particles which provide the core in the composite particle to be formed. The layer-by-layer process for producing mouldings is preferably selective laser sintering. Because of the firm bond between polymer and filler, the powder according to the present invention is no longer subject to the problems of demixing, and this leads to an improvement in consistency of mechanical properties in the moulding produced from the powder. Since demixing no longer occurs in the powder according to the invention, it is possible to use the said powder in construction processes to produce uniform components and components with unifomn quality. The durably unifonn constitution resulting from the firm bond between polymer and core particle significantly improves the recyclability of the powder, even when a plurality of stages are involved. There are also advantages in the use of the powders according to the invention: the powders according to the invention can be stored, transported and used in larger packaging units without any possibility of demixing. Feed quantities of the product can therefore also be greater during the laser sintering process, i.e. more powder can be charged to the sample feed container, and/or the dimensions of the sample feed container can be greater, without any resultant adverse effect on the quality of the resultant components. Furthemnore, fluidization in the feed does not lead to the demixing that is relatively frequently observed in systems of the prior art. Because the powders of the present invention have an exterior shell made of polymer, the introduction of energy by the laser is also more uniform. In powders of the prior art, the laser sometimes encounters a polymer particle and sometimes encounters a filler particle. As a function of filler type, the result can vary in extreme cases from almost complete absorption to almost complete reflection of the energy. Powders according to the present invention advantageously avoid these problems. Surprisingly, it has now been found tliat, by using core particles made of an inorganic material with dso median diameter of greater than 1 to 70 pm as reinforcing material firmly bonded to the polymer (composite particles), it is possible, through a layer-by-layer process (in which regions of the respective powder layer are selectively melted) to produce mouldings which have advantages in relation to density and susceptibility to warpage and with this have better properties in relation to consistency of processing than those made of a reinforced polymer powder of the prior art. It is preferable that reinforcement of the moulding is achieved, or an improvement in thermal conductivity, and, respectively, as a function of the type of core particles used, flame retardancy. In one prefen^ed embodiment, the core particles to be coated with the precipitatable polymer have been selected from the group of silicon dioxide, polyphosphates, phosphinates, metal nitrides, semimetal nitrides, aluminium nitrides, boron nitride, boron carbide, metal oxides, for example Ai203, mixed oxides, spinels, metal, ceramic and mixtures thereof. It is further preferable that the core particles to be coated with the precipitatable polymer have been selected from the group of silicon dioxide, polyphosphates, phosphinates, boron carbide, metal oxides, for example AI2O3, mixed oxides, spinels and mixtures thereof. It is moreover possible that further fillers or filler mixtures are present alongside the core particles mentioned in the composite particle. The respective core particles can take the following forms: spherical, lamellar or elongate. The respective core particles can moreover be sharp-edged, rounded or smooth. The core particles mentioned can also optionally have been coated with sizes prior to application of the polymer to be precipitated. The said core particles provide the core in the composite particle. The powder according to the present invention preferably has a core-shell structure. In another preferred embodiment, the average thickness of the coating made of the precipitated polymer is 1.5 pm or more, preferably 2, 3, 5, 10, 15, 20, 25, 30, 50, or 75 pm or more. In another preferred embodiment, the also median diameter of the core particles (core of the composite particle) is from 1 to 60 pm, preferably from 1 to 50 pm, with preference from 1 to 40 pm, more preferably from 1 to 30 pm, still more preferably from 1 to 25 pm, particularly preferably from 1 to 20 pm and very particularly preferably from 1 to 10 pm. Suitable particle size distributions can be ensured by known methods, e.g. sieving or sifting. In an alternative embodiment, the also median diameter of the core particles (core of the composite particle) is from 10 to 60 µm, preferably from 10 to 50 µm, with preference from 10 to 40 µm, more preferably from 10 to 30 µm, still more preferably from 10 to 25 µm and particularly preferably from 10 to 20 µm . Again in this preferred embodiment, the core particles to be coated with the precipitatable polymer have been selected from the group of silicon dioxide, polyphosphates, phosphinates, aluminium nitride, boron nitride, boron carbide, metal oxides, e.g. AI2O3, mixed oxides, spinels, metal and ceramic and mixtures thereof. It is moreover preferable that the also median diameter of the composite particles is from 20 to 150 pm, with preference from 20 to 120 pm, preferably from 20 to 100 pm, more preferably from 25 to 80 pm and particularly preferably from 25 to 70 pm. The ratio of the dso median diameter of the composite particles to the dso median diameter of the core particles is preferably from 1.15 to 30, with preference from 1.2 to 30, more preferably from 1.5 to 25; still more preferably from 1.5 to 15, particularly preferably from 1.5 to 12 and very particularly preferably from 1.5 to 10. The ratio by weight of the polymer to the core particles, based on the entirety of the composite particles, is preferably from 0.1 to 30, with preference from 1.0 to 20.0 and more preferably from 1.3 to 10.0. In another preferred embodiment, the BET specific surface area of the powder according to the invention is in the range from 1 to 60 m^/g, with preference from 3 to 50 m^/g, more preferably from 3 to 40 m^/g; particularly preferably from 3 to 30 m^/g, still more preferably from 3 to 20 m^/g and very particularly preferably from 3 to 12 m/g. The bulk density BD of the powder according to the invention is moreover in the range from 120 to 700 g/l, with preference from 250 to 450 g/l. The precipitated or precipitatable polymer is a polymer which can be dissolved in a liquid medium comprising a solvent and which precipitates in the form of a completely or partially insoluble deposit in the form of flakes or droplets, or in crystalline form, as a result of changes of certain parameters, e.g. temperature, pressure, solvent content, non-solvents, anti-solvents, or precipitants. The type of solvent and the solvent content depend here on the polymer, as also do the other parameters for dissolving or precipitating the appropriate polymer. The precipitatable polymer has preferably been selected from polyolefins, polyethylene, polypropylene, polyvinyl chloride, polyacetal, polystyrene, polyimides, polysulphones, poly(N-methylmethacrylimides) (PMMI), polymethyl methacrylate (PMMA), polyvinylidene fluorides (PVDF), ionomer, polyether ketones, polyaryl ether ketones, polyamide, copolyamide and mixtures thereof, in particular mixtures of homo- and copolyamide. In another embodiment, the precipitatable polymer for coating the core particles is obtained through precipitation of at least one polyamide of the AABB type or through joint precipitation of at least one polyamide of the AB type and at least one polyamide of the AABB type. Preference is given here to co-precipitated polyamides, where at least nylon-11 or nylon-12 and at least one polyamide based on PA1010, PA1012, PA1212 or PA1013 is present. The following precipitatable polymers may be mentioned by way of example: polyolefins and polyethylene can be dissolved by way of example in toluene, xylene and/or 1,2,4-trichlorobenzene. Polypropylene can be dissolved by way of example in toluene and/or xylene. Polyvinyl chloride can be dissolved by way of example in acetone. Polyacetal can be dissolved by way of example in DMF, DMAc and/or NMP. Polystyrene can be dissolved by way of example in toluene. Polyimides can be dissolved by way of example in NMP. Polysulphones can be dissolved by way of example in sulpholane. Poly(N-methylmethacrylimides) (PMMI) can be dissolved by way of example in DMAc and/or NMP. Polymethyl methacrylate (PMMA) can be dissolved by way of example in acetone. Polyvinylidene fluorides can be dissolved in N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc) and/or cyclohexanone. Polyether ketones and polyaryl ether ketones can be dissolved by way of example in diphenyl sulphone and/or in sulpholane. Polyamides can be dissolved in an alcoholic medium, preferably an ethanol-water mixture. As explained above, it is sometimes also necessary to adjust parameters such as temperature and pressure in order to dissolve a given polymer. Once the relevant polymer has been dissolved, this is precipitated in the presence of the core particles, in order to coat the core particles completely or partially with the relevant precipitated polymer. The precipitation of the polymer can be initiated and/or accelerated by changing the pressure, changing the temperature, changing (reducing) the concentration of the solvent, and optionally adding a non-solvent, anti-solvent and/or precipitant. In the case of amorphous polymers, such as polystyrene, polysulphones, PMMI, PMMA, and ionomer, it is necessary to add a non-solvent to precipitate the relevant polymer. The precipitatable polymer is preferably a polyamide which has at least 8 carbon atoms per carbonamide group. The polymer is particularly preferably a polyamide which has 10 or more carbon atoms per carbonamide group. The polymer is very particularly preferably a polyamide selected from nylon-6,12 (PA 612), nylon-11 (PA 11) and nylon-12 (PA 12). The production process for the polyamides that can be used in the sinter powders according to the invention is well-known and, for the production of PA 12, can be found by way of example in the documents DE 29 06 647, DE 35 10 687, DE 35 10 691 and DE 44 21 454. The granulated polyamide material required can be purchased from various producers, and by way of example granulated nylon-12 material is available with trade name VESTAMID from Evonik Industries AG. It is particularly preferable that the precipitatable polymer is nylon-12. It is moreover possible to use the corresponding copolyamides or mixtures of homo- and copolyamides which comprise at least 70 percent by weight of the units mentioned. Accordingly, they can comprise, as comonomers, from 0 to 30 percent by weight of one or more comonomers, such as caprolactam, hexamethylenediamine, 2-methyl-1,5-pentanediamine, 1,8-octamethylenediamine, dodecamethylenediamine, isophoronediamine, trimethylhexamethylenediamine, adipic acid, suberic acid, azeleic acid, sebacic acid, dodecanedioic acid, aminoundecanoic acid. The homo- and copolyamides mentioned, termed polyamides hereinafter, are used in the form of granulated materials or ground material, where the relative solution viscosity of these is from 1.5 to 2.0 (measured in 0.5% m-cresol solution at 25°C in accordance with DIN 53 727), preferably from 1.70 to 1.95. They can be produced by polycondensation, or hydrolytic or acidolytic or activated anionic polymerization, by known processes. It is preferable to use unregulated polyamides having NH2/COOH end group ratios of from 40/60 to 60/40. However, it is also advantageously possible to use regulated polyamides and specifically preferably those in which the NH2/COOH end group ratio is 90:10 and 80:20 or 10:90 and 20:80. In another prefeaecl embodiment, the density of the core particles is greater or not more than 20%, with preference not more than 15%, more preferably not more than 10% and particularly preferably not more than 5% smaller than the density of the solvent used for the precipitation of the polymer. It is particularly preferable to use an alkanol (for example: methanol, ethanol, propanol or butanol), preferably ethanol, as solvent for the precipitation of the polymer in the presence of the core particles, vi/here the density of the core particles is greater or not more than 20%, with preference not more than 15%, more preferably not more than 10% and particularly preferably not more than 5% smaller than the density of the alkanol, preferably of ethanol. The powder can comprise the composite particles mentioned alone or together with, admixed therewith In uncompacted fomi, (dry-blend) fillers, and/or auxiliaries. The proportion of the composite particles In the powder is at least 50% by weight, with preference at least 80% by weight, preferably at least 90% by weight, particulariy preferably at least 95% by weight and very particulariy preferably at least 99% by weight. The powders according to the invention can moreover comprise auxiliaries and/or other organic or Inorganic pigments. These auxiliaries can by way of example be powder-flow aids, e.g. precipitated and/or fumed silicas. Precipitated silicas are available by way of example with product name AEROSIL® with various specifications from Evonik Industries AG. It Is preferable that the powder according to the invention comprises less than 3% by weight of these auxiliaries, with preference from 0.001 to 2% by weight and very particulariy preferably from 0.025 to 1% by weight, based on the entirety of the polymers present. In order to improve processabillty or for further modification of the powder according to the Invention, inorganic foreign pigments, e.g. transition metal oxides, stabilizers, e.g. phenols. In particular sterically hindered phenols, fiow aids and powder-fiow aids, e.g. fumed silicas, can be added thereto. The amount of the said substances added to the polymers, based on the total weight of polymers in the polymer powder. Is preferably such as to provide compliance with the concentrations stated for auxiliaries for the powder according to the invention. Ideal properties in the further processing of the powder are achieved when the melting point of the polymer In the first heating procedure Is greater than in the second heating procedure, measured by differential scanning calorimetry (DSC); and when the enthalpy of fusion of the polymer in the first heating procedure is at least 50% greater than in the second heating procedure, measured by differential scanning calorimetry (DSC). With this, the polymer content (of the shell or of the coating) of the composite particles has higher crystallinity when compared with other powders which can be produced by processes other than co-precipitation of a dissolved polymer with particles. A particularly suitable material for the laser sintering process is a nylon-12 which has a melting point of from 185 to ISS^C, with preference from 186 to 188°C, an enthalpy effusion of 112 +/-17 /mol, with preference from 100 to 125 l 10 pm AI2O3 (Martoxid® DN 206) 5-7 pm AI2O3 (Martoxid® MDLS-6) 3-4 pm AI2O3 (Martoxid® MZS-1) 1.5-1.9 pm Ammonium polyphosphate (Exollt® AP 422) 18.33 pm Phosphinate (Exolit® OP1230) 11 pm Si02 (Aeroperl® 300/30) 8 pm Stainless steel flakes 31 pm AS081 aluminium powder 28 pm In this example, the precipitation conditions were altered in the following way In comparison with Example 1: Precipitation temperature: 108°C Precipitation time: 150 min Stirrer rotation rate: from 39 to 82 rpm Table 2 collates the characterization (bulk density, diameter and BET surface area) of the powders produced in accordance with Example 2. Alongside this, Table 2 also gives the amounts used of polyamide, core particles and ethanol, and also the stin-er rotation rate used in the process. Table 2: Characterization of the powders produced in accordance with Example 2 Patent claims 1. Powder for use in a layer-by-layer process for producing mouldings by selectively melting regions of the respective powder layer through) introduction of electromagnetic energy, comprising composite particles which are produced from core particles completely or partially coated with a precipitated polymer, where the core particles are inorganic core particles, with the exception of titanium dioxide, and where the d50 median diameter of the inorganic core particles is from 1 to 70 µm. 2. Powder according to Claim 1, characterized in that the inorganic core particles have been elected from the group of silicon dioxide, polyphosphates, phosphinates, metal nitrides, semimetal nitrides, boron carbide, metal oxides, mixed oxides, spinels, metal and ceramic. 3. Powder according to Claim 1 or 2, characterized in that the thickness of the coating made of the precipitated polymer is 1.5 um or more, preferably 2, 3, 5, 10, 15, 20, 25, 30, 50, or 75 um or more. 4. Powder according to any one of Claims 1 to 3, characterized in that the d50 median diameter of the core particles is from 1 to 60 , preferably from 1 to 50 m, with preference from 1 to 40 Mm, more preferably from 1 to 30 pm, still more preferably from 1 to 25 um, particularly preferably from 1 to 20 pm and very particularly preferably from 1 to 10 pm. 5. Powder according to any one of Claims 1 to 4, characterized in that the d50 median diameter of the composite particles is from 20 to 150 pm, with preference from 20 to 120 pm, preferably from 20 to 100 pm, more preferably from 25 to 80 pm and particularly preferably from 25 to 70 pm. 6. Powder according to any one of Claims 1 to 5, characterized in that the ratio of the d50 median diameter of the composite particles to the d50 median diameter of the core particles is from 1.15 to 30, preferably from 1.2 to 30, with preference from 1.5 to 25; preferably from 1.5 to 15, more preferably from 1.5 to 12 and particularly preferably from 1.5 to 10. Powder according to any one of Claims 1 to 6, characterized in that the ratio, based on weight, of the polymer to the core particles, based on the entirety of the composite particles, is from 0.1 to 30, with preference from 1.0 to 20.0 and more preferably from 1.3 to 10.0. Powder according to any one of Claims 1 to 7, characterized in that the precipitatable polymer has been selected from polyolefins, polyethylene, polypropylene, polyvinyl chloride, polyacetal, polystyrene, polyimides, polysulphones, poly(N-methyl-methacrylimides) (PMMI), polymethyl methacrylate (PMMA), polyvinylidene fluorides (PVDF), ionomer, polyether ketones, polyaryl ether ketones, polyamide, copolyamide or a mixture thereof, in particular a mixture of homo- and copolyamide; preferably polyamide, particularly preferably nylon-11, nylon-12 and polyamides having more than 12 aliphatically bonded carbon atoms per carbonamide group. Powder according to any one of Claims 1 to 8, characterized in that the density of the core particles is greater or not more than 20%, with preference not more than 15%, more preferably not more than 10% and particularly preferably not more than 5% smaller than the density of the solvent used for the precipitation of the polymer. Powder according to any one of Claims 1 to 8, characterized in that the density of the core particles is greater or not more than 20%, with preference not more than 15%, more preferably not more than 10% and particularly preferably not more than 5% smaller than the density of ethanol. 1. Powder according to any one of Claims 1 to 10, characterized in that the proportion of the composite particles in the powder is at least 50% by weight, with preference at least 80% by weight, preferably at least 90% by weight, particularly preferably at least 95% by weight and very particularly preferably at least 99% by weight. I. Powder according to any one of Claims 1 to 11, characterized in that the melting point of the polymer in the first heating procedure is greater than in the second heating procedure. measured by differential scanning calorimetry (DSC). 13. Powder according to any one of Claims 1 to 11, characterized in that the enthalpy of fusion of the polymer in the first heating procedure is at least 50% greater than in the second heating procedure, measured by differential scanning calorimetry (DSC). 14. Process for producing powders, in particular as defined in any one of Claims 1 to 13, where, in order to produce an at least partial solution, a polymer is brought into contact, in the presence of core particles, with exposure to pressure and/or heat, with a medium comprising solvent which dissolves the polymer, and then the polymer is precipitated from the at least partial solution, and composite particles are obtained which are produced by core particles coated entirely or partially with a precipitated polymer, where the core particles are inorganic core particles, with the exception of titanium dioxide, and where the also median diameter of the inorganic core particles is from 1 to 70 µm. 15. Process for producing mouldings by a layer-by-layer process in which regions of the respective powder layer are selectively melted through introduction of electromagnetic energy, where the selectivity is achieved by applying susceptors, inhibitors, or absorbers or by masks, where a powder according to at least one of Claims 1 to 13 is used, in particular a powder which comprises composite particles which are produced by core particles coated entirely or partially with a precipitated polymer, where the core particles are inorganic core particles, with the exception of titanium dioxide, and where the dso median diameter of the inorganic core particles is from 1 to 70 µm. 16. Use of the powder according to any one of Claims 1 to 13 in a process according to Claim 15. 17. Moulding which has been obtained by the process according to Claim 15 with use of the powder according to any one of Claims 1 to 13. 18. Powder obtainable according to a process according to Claim 14.