TECHNICAL FIELD [0001] A subject of the present invention is a process for the transportation and/or storage of road bitumen. The present invention also relates to the use of bitumen blocks according to the invention as road binder, and also to their use in manufacturing mixes. PRIOR ART [0002] The vast majority of bitumen is used in construction, mainly for the manufacture of road carriageways or in industry, for example for roofing applications. It is generally in the form of a black material which is highly viscous, or even solid at ambient temperature, and which liquefies when heated. Generally, bitumen is stored and transported under hot conditions, in bulk, in tanker trucks or by boats at high temperatures of about 120°C to 160°C. However, the storage and transportation of bitumen under hot conditions has certain drawbacks. Firstly, the transportation of bitumen under hot conditions in liquid form is considered to be dangerous and it is highly restricted from a regulatory point of view. This mode of transportation does not present particular difficulties when the transportation equipment and infrastructures are in good condition. If this is not the case, it can become problematic: if the tanker truck is not sufficiently lagged, the bitumen may become viscous during an excessively long trip. Bitumen delivery distances are therefore limited. Secondly, maintaining bitumen at high temperatures in tanks or in tanker trucks consumes energy. In addition, maintaining the temperature of the bitumen for a given period may affect the properties of the bitumen, especially the ageing properties and thus change the final performance properties of the mix. [0003] In order to overcome the problems of transporting and storing bitumen under hot conditions, packaging solutions for transporting and storing bitumens under cold conditions have been developed. This mode of transportation of bitumen in packaging under cold conditions represents only a minimal fraction of the amounts transported throughout the world, but it corresponds to very real needs for geographic regions which are difficult and expensive to access using conventional transportation means. [0004] By way of example, mention may be made of transporting bitumen at ambient temperature in metal barrels. This means is increasingly questionable from an environmental point of view since the cold bitumen stored in the barrels must be reheated before it is used as road binder. However, this operation is difficult to carry out for this type of packaging and the barrels constitute waste after use. Furthermore, the storage of bitumen under cold conditions in barrels results in losses since the bitumen is very viscous and a part of the product remains on the walls of the barrel when the bitumen is transferred into the tanks of mix production units. With regard to the handling and transportation of bituminous products in these barrels, they can prove to be difficult and dangerous if the specialized equipment for handling the barrels is not available in the transporters or at the site where the bitumen is used. [0005] The packaging of bituminous products in paper bags or in thermoplastic material, such as polypropylene or polyethylene, has been the subject of recent developments. By way of example, patent application US 2011/0290695 describes a system for dispensing and packaging bituminous products in block form. Each bitumen block is surrounded by a film of bituminous composition, consisting of approximately 10 to 30% by weight of natural bitumen and approximately 5 to 25% by weight of a synthetic elastomer and copolymers. The film of bituminous composition is melted with the bituminous product and is entirely compatible with the melted bitumen. [0006] However, it has been observed that bituminous products packaged in the form of paper bags or bags made of thermoplastic material may creep during the handling, storage and transportation thereof, because the bags or films made of thermoplastic material may be pierced, increasing the risks of deformation and of leakage, especially when the external temperature is high. When the bags or films made from thermoplastic material are pierced, the bitumen flows and the bags or blocks surrounded by film made of thermoplastic material stick to one another. Handling of the bags or blocks surrounded by film made of thermoplastic material degraded in this way becomes impossible, which makes them unusable. [0007] Despite the recent development in the packaging of bituminous products, there is still a need to find a solution for storing and transporting road bitumen and bituminous materials under cold conditions, which makes it possible to overcome the drawbacks mentioned above. [0008] In particular, the aim of the present invention is to propose an improved process for transportation and/or storage under cold conditions. [0009] Another aim of the invention is to propose a process enabling easy handling of the road bitumen during handling operations. [0010] Another aim of the invention is to propose an ecological and economical method for transporting road bitumen and avoiding the use of additional means for maintaining the temperature of said bitumen during transport and/or storage. [0011] Another aim of the invention is to propose an ecological method which makes it possible to minimise the presence of waste and/or residues. SUBJECT OF THE INVENTION [0012] The subject of the invention relates to a process for the transportation and/or storage of road bitumen under cold conditions, said bitumen being transported and/or stored in bitumen block form and comprising at least one chemical additive chosen from: a compound of general formula (I): R1-(COOH)2 in which R1 is a linear or branched, saturated or unsaturated hydrocarbon-based chain comprising from 4 to 68 carbon atoms, preferably from 4 to 54 carbon atoms, more preferentially from 4 to 36 carbon atoms, and z is an integer ranging from 1 to 4, preferably from 2 to 4, and a compound of general formula (II): R-(NH)nCONH-{X)m-NHCO(NH)n-R' in which: R and R', which are identical or different, contain a saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbon-based chain having from 1 to 22 carbon atoms and optionally comprising heteroatoms and/or rings having from 3 to 12 atoms and/or heterocycles having from 3 to 12 atoms; X contains a saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbon-based chain having from 1 to 22 carbon atoms and optionally comprising one or more heteroatoms and/or rings having from 3 to 12 atoms and/or heterocycles having from 3 to 12 atoms; n and m are integers having, independently of one another, a value of 0 or of 1. [0013] The term "road bitumen" is intended to mean bituminous compositions consisting of one or more bitumen bases and comprising one or more chemical additives, said compositions being intended for a road application. [0014] According to one particular embodiment, the road bitumen is transported and/or stored at a temperature of less than 100°C. In particular, the temperature of transportation and/or storage corresponds to the ambient temperature. "Ambient temperature" is intended to mean the temperature which is reached during transportation and/or storage of the bitumen according to the invention, without said bitumen being heated by any type of process. Thus, the ambient temperature may reach high temperatures, less than 100°C during summer periods, in particular in geographical regions with a hot climate. [0015] According to a particular preferential embodiment, the bitumen is transported and/or stored at a temperature of between 20°C and 90°C, preferably of between 20°C and 80°C, more preferentially of between 40°C and 80°C, even more preferentially of between 50°C and 70°C, even more preferentially of between 40°C and 60°C. [0016] According to a particular embodiment, the chemical additive is a compound of general formula (I): R1-{COOH)z in which R1 is a linear or branched, saturated or unsaturated hydrocarbon-based chain comprising from 4 to 68 carbon atoms, preferably from 4 to 54 carbon atoms, more preferentially from 4 to 36 carbon atoms, and z is an integer ranging from 1 to 4, preferably from 2 to 4. [0017] According to a particular preferential embodiment, the compound is a diacid of general formula HOOC-CwH2w-COOH, in which w is an integer ranging from 4 to 22, preferably from 4 to 12. [0018] Advantageously, the compound is a diacid chosen from the group consisting of adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, 1,2-dodecanedioic acid and tetradecanedioic acid. [0019] According to a particular embodiment, the chemical additive is a compound of general formula (II): R-(NH)nCONH-(X)m-NHCO(NH)n-R' in which: R and R', which are identical or different, contain a saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbon-based chain having from 1 to 22 carbon atoms and optionally comprising heteroatoms and/or rings having from 3 to 12 atoms and/or heterocycles having from 3 to 12 atoms; X contains a saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbon-based chain having from 1 to 22 carbon atoms and optionally comprising one or more heteroatoms and/or rings having from 3 to 12 atoms and/or heterocycles having from 3 to 12 atoms; n and m are integers having, independently of one another, a value of 0 or of 1. [0020J According to a particular preferential embodiment, the compound comprises a hydrazide unit when n and m have a value of 0. [0021] Advantageously, the R and/or R' groups, which are identical or different, comprise one or more aromatic monocyclic or polycyclic rings or heterocycles, optionally substituted by one or more hydroxyl functions and/or one or more saturated, linear or branched hydrocarbon-based chains, having from 1 to 6 carbon atoms. [0022] According to a particular preferential embodiment, the compound comprises two amide units when n has a value of 0 and m has a value of 1. [0023] According to a particular preferential embodiment, the R and/or R' groups, which are identical or different, comprise an aliphatic hydrocarbon-based chain having 4 to 22 carbon atoms, especially chosen from the groups C4H9, C5H11, C9H19, C11H23, C12H25, C17H35, C18H37, C21H43, C22H45. [0024] Advantageously, the X group comprises an aliphatic hydrocarbon-based chain having 1 to 2 carbon atoms. [0025] According to one particular embodiment, the road bitumen comprises from 0.1% to 5% by weight, preferably from 0.5% to 4% by weight, more preferentially from 0.5% to 2.5% by weight of the chemical additive relative to the total weight of said bitumen. [0026] According to another particular embodiment, the road bitumen comprises from 5% to 30% by weight, preferably from 6% to 28% by weight, more preferentially from 7% to 26% by weight of the chemical additive relative to the total weight of said bitumen. [0027] According to one particular embodiment, the road bitumen comprises between 0.05% and 15% by weight, preferably between 0.1% and 10% by weight, more preferentially between 0.5% and 6% by weight of an olefinic polymer adjuvant. [0028] A subject of the invention also relates to the use of bitumen block as road binder, said bitumen block consisting of road bitumen as described in the process according to the invention. [0029] According to one particular embodiment, the bitumen block is used for manufacturing mixes, said bitumen block consisting of road bitumen as described in the process according to the invention. DETAILED DESCRIPTION [0030] According to one particular embodiment, a road bitumen is prepared by bringing into contact: at least one bitumen base, between 0.1% and 5% by weight, preferably between 0.5% and 4% by weight and more preferably between 0.5% and 2.5% by weight of a chemical additive. [0031] According to another particular embodiment, a road bitumen is prepared by bringing into contact: at least one bitumen base, between 5% and 30% by weight, preferably between 6% and 28% by weight, more preferentially between 7% and 26% by weight of the chemical additive relative to the total weight of said bitumen. [0032] Within the meaning of the invention, when the road bitumen comprises between 5% and 30% by weight, preferably between 6% and 28% by weight, more preferentially between 7% and 26% by weight of the chemical additive relative to the total weight of said bitumen, said road bitumen may also be referred to as concentrated road bitumen. [0033] Throughout the remainder of the description, the percentages by weight are calculated relative to the total weight of the bitumen. [0034] The operation is carried out at manufacturing temperatures of between 100°C and 200°C, preferably between 140°C and 200°C and more preferentially between 140°Cand 170°C, and with stirring for a period of time of at least 10 minutes, preferably of between 30 minutes and 10 hours and more preferentially between 1 hour and 6 hours. The term "manufacturing temperature" is intended to mean the temperature to which the bitumen base is heated before mixing and also the mixing temperature. The heating temperature and time vary according to the amount of bitumen used and are defined by standard NF EN 12594. [0035] Among the bitumen bases that can be used according to the invention, mention may be made first of all of bitumens of natural origin, those contained in natural bitumen or natural asphalt deposits or bituminous sands and bitumens originating from the refining of crude oil. The bitumen bases according to the invention are advantageously chosen from bitumen bases originating from the refining of crude oil. The bitumen bases may be chosen from bitumen bases or mixtures of bitumen bases originating from the refining of crude oil, in particular bitumen bases containing asphaltenes. The bitumen bases can be obtained by conventional processes for manufacturing bitumen bases in refining, in particular by direct distillation and/or vacuum distillation of oil. These bitumen bases can optionally be visbroken and/or deasphalted and/or air-rectified. The various bitumen bases obtained by means of refining processes can be combined with one another to obtain the best technical compromise. The bitumen base may also be a bitumen base from recycling. The bitumen bases may be bitumen bases of hard grade or of soft grade. The bitumen bases according to the invention have a penetrability, measured at 25°C according to standard EN 1426, of between 5 and 300 1/10 mm, preferably between 10 and 100 1/10 mm, more preferentially between 30 and 100 1/10 mm. [0036] According to a particular embodiment, the chemical additive may be a compound of the following general formula (I): R1-(COOH)z in which R1 is a linear or branched, saturated or unsaturated hydrocarbon-based chain comprising from 4 to 68 carbon atoms, preferably from 4 to 54 carbon atoms, more preferentially from 4 to 36 carbon atoms, and z is an integer ranging from 1 to 4, preferably from 2 to 4, more preferentially equal to 2. [0037] The chemical additives corresponding to the formula (I) may advantageously be monoacids (z=1), diacids (z=2), triacids (z=3) or tetracids (z=4). The preferred chemical additives are diacids with z=2. Similarly, the R1 group is preferably a linear and saturated hydrocarbon-based chain of formula CWH2W with w an integer ranging from 4 to 22, preferably from 4 to 12. [0038] The chemical additives have, in particular, the general formula HOOC-CwH2w-COOH, where w is an integer ranging from 4 to 22, preferably from 4 to 12. These chemical additives correspond to the above formula (I) in which z = 2 and R1= CwH2w-[0039] The preferred diacids are as follows: adipic acid or 1,6,-hexanedioic acid, with w=4 pimelic acid or 1,7,-heptanedioic acid, with w=5 suberic acid or 1,8,-octanedioic acid, with w=6 azelaic acid or 1,9,-nonanedioic acid, with w=7 sebacicacid or 1,10,-decanedioicacid, with w=8 undecanedioic acid, with w=9 1,2-dodecanedioic acid, with w=10 tetradecanedioic acid, with w=12. [0040] Advantageously, the diacid is sebacic acid. [0041] The diacids may also be diacid dimers of unsaturated fatty acid(s), i.e. dimers formed from at least one unsaturated fatty acid, for example from a single unsaturated fatty acid or from two different unsaturated fatty acids. The diacid dimers of unsaturated fatty acid(s) are conventionally obtained by intermolecular dimerization reaction of at least one unsaturated fatty acid (Diels-Alder reaction for example). Preferably, a single type of unsaturated fatty acid is dimerized. They derive in particular from the dimerization of an unsaturated fatty acid which is especially C8 to C34, especially C12 to C22, in particular Ci6 to C20, and more particularly Ci8. A preferred fatty acid dimer is obtained by dimerization of linoleic acid, it being possible for the latter to be subsequently partially or totally hydrogenated. Another preferred fatty acid dimer has the formula HOOC-(CH2)7-CH=CH-(CH2)7-COOH. Another preferred fatty acid dimer is obtained by dimerization of methyl linoleate. In the same way, it is possible to have fatty acid triacids and fatty acid tetraacids, obtained respectively by trimerization and tetramerization of at least one fatty acid. [0042] According to another particular embodiment, the chemical additive may be a compound of the following general formula (II): R-(NH)nCONH-{X)m-NHCO(NH)n-R' in which: R and R\ which are identical or different, contain a saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbon-based chain having from 1 to 22 carbon atoms and optionally comprising heteroatoms and/or rings having from 3 to 12 atoms and/or heterocycles having from 3 to 12 atoms; X contains a saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbon-based chain having from 1 to 22 carbon atoms and optionally comprising one or more heteroatoms and/or rings having from 3 to 12 atoms and/or heterocycles having from 3 to 12 atoms; n and m are integers having, independently of one another, a value of 0 or of 1. [0043] According to a variant of the invention, the integer m has a value of 0. In this particular case, the R-(NH)nCONH and NHCO(NH)n-R' groups are covalently bonded to one another via a hydrazide linkage CONH-NHCO. The R and/or R' groups, which are identical or different, then comprise a group chosen from at least one hydrocarbon-based chain having at least 4 carbon atoms, at least one aliphatic ring having 3 to 8 atoms, at least one aliphatic, partially aromatic or entirely aromatic fused polycyclic system, each ring comprising 5 or 6 atoms, taken alone or in a mixture. [0044] Preferably, the R and/or R' groups, which are identical or different, are linear saturated hydrocarbon-based chains comprising from 4 to 22 carbon atoms. Among the preferred linear saturated hydrocarbon-based chains, mention may be made of the C4H9, C5H11, CgHig, C11H23, C12H25, C-17H35, C18H37, C21H43C22H45 groups. [0045] According to another variant of the invention, the integer m has a value of 1. The R group, R' group and/or the X group then comprise a group chosen from at least one hydrocarbon-based chain having at least 4 carbon atoms, at least one aliphatic ring having 3 to 8 atoms, at least one aliphatic, partially aromatic or entirely aromatic fused polycyclic system, each ring comprising 5 or 6 atoms, taken alone or in a mixture. [0046] Preferably, the X group represents a saturated, linear hydrocarbon-based chain comprising from 1 to 22 carbon atoms. Preferably, the X group is chosen from C2H4 and C3H6 groups. [0047] The X group may also be a cyclohexyl group or a phenyl group, the R-(NH)nCONH- and NHCO(NH)n-R'- radicals may then be in the ortho, meta or para position. Moreover, they may be in the cis or trans position with respect to one another. Furthermore, when the X radical is cyclic, this ring may be substituted with groups other than the two main groups R-(NH)nCONH- and -NHCO Test of resistance to water stripping according to standard NF EN 12697-12 (method B) <2) Test of resistance to rutting according to standard NF EN 12697-22 {3) Test of low temperature cracking according to standard NF EN 12697-46 (4) Modulus of the mixes according to standard NF EN 12697-26 (5) Fatigue of the mixes according to standard NF EN 12697-24 [00132] The mixes produced with a bitumen according to the invention as road binder have identical properties compared to the mixes of the prior art E0 and E5. The addition of one or more additives according to the invention to a bitumen does not affect the properties of the mixes produced from such a bitumen. [00133] Thus, for each test carried out, the void contents are comparable for E0 and E3 and also for E5 and E9l which means equivalent ability to be handled. This observation is consistent since the bitumens B0 and B3 and also the bitumens B5 and B9 have comparable viscosities between 120X and 160aC (cf. Table 3). In addition, the resistance values r and R are higher for the mixes E3 and E9 than for the mixes E0 and E5. [00134] The modulus and fatigue properties of the mix E3 and E9 are very satisfactory compared, respectively, to the mixes E0 and E5. We Claim: 1. Process for the transportation and/or storage of road bitumen under cold conditions, said bitumen being transported and/or stored in bitumen block form and comprising at least one chemical additive chosen from: • a compound of general formula (I): R1-(COOH)z in which R1 is a linear or branched, saturated or unsaturated hydrocarbon-based chain comprising from 4 to 68 carbon atoms, preferably from 4 to 54 carbon atoms, more preferentially from 4 to 36 carbon atoms, and z is an integer ranging from 1 to 4, preferably from 2 to 4, or • a compound of general formula (II): R-(NH)nCONH-(X)m-NHCO(NH)n-R' in which: R and R', which are identical or different, contain a saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbon-based chain having from 1 to 22 carbon atoms and optionally comprising heteroatoms and/or rings having from 3 to 12 atoms and/or heterocycles having from 3 to 12 atoms; X contains a saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbon-based chain having from 1 to 22 carbon atoms and optionally comprising one or more heteroatoms and/or rings having from 3 to 12 atoms and/or heterocycles having from 3 to 12 atoms; n and m are integers having, independently of one another, a value of 0 or of 1. 2. Process according to Claim 1, in which said road bitumen is transported and/or stored at a temperature of less than 100°C, preferably at an ambient temperature of less than 100°C. 3. Process according to either of Claims 1 and 2, in which said road bitumen is transported and/or stored at an ambient temperature of between 20°C and 90°C, preferably of between 20°C and 80°C, more preferentially of between 40°C and 80°C, even more preferentially of between 50°C and 70°C, even more preferentially of between 40°C and 60°C. 4. Process according to any one of Claims 1 to 3, in which the chemical additive is a compound of general formula (I): R1-(COOH)z in which R1 is a linear or branched, saturated or unsaturated hydrocarbon-based chain comprising from 4 to 68 carbon atoms, preferably from 4 to 54 carbon atoms, more preferentially from 4 to 36 carbon atoms, and z is an integer ranging from 1 to 4, preferably from 2 to 4. 5. Process according to Claim 4, in which said compound is a diacid of general formula HOOC-CwH2w-COOH, in which w is an integer ranging from 4 to 22, preferably from 4 to 12. 6. Process according to either of Claims 4 and 5, in which said compound is a diacid chosen from the group consisting of adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, 1,2-dodecanedioic acid and tetradecanedioic acid. 7. Process according to any one of Claims 1 to 3, in which the chemical additive is a compound of general formula (II): R-(NH)nCONH-(X)m-NHCO(NH)n-R' in which: R and R', which are identical or different, contain a saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbon-based chain having from 1 to 22 carbon atoms and optionally comprising heteroatoms and/or rings having from 3 to 12 atoms and/or heterocycles having from 3 to 12 atoms; X contains a saturated or unsaturated, linear or branched, cyclic or acyclic hydrocarbon-based chain having from 1 to 22 carbon atoms and optionally comprising one or more heteroatoms and/or rings having from 3 to 12 atoms and/or heterocycles having from 3 to 12 atoms; n and m are integers having, independently of one another, a value of 0 or of 1. 8. Process according to Claim 7, in which said compound comprises a hydrazide unit when n and m have a value of 0. 9. Process according to claim 8, in which said R and/or R' groups, which are identical or different, comprise one or more aromatic monocyclic or polycyclic rings or heterocycles, optionally substituted by one or more hydroxyl functions and/or one or more saturated, linear or branched hydrocarbon-based chains, having from 1 to 6 carbon atoms. 10. Process according to Claim 7, in which said compound comprises two amide units when n has a value of 0 and m has a value of 1. 11. Process according to any one of Claims 7 to 10, in which said R and/or R' groups, which are identical or different, comprise an aliphatic hydrocarbon-based chain having 4 to 22 carbon atoms, especially chosen from the groups C4H9, C5H11, C9H19, C11H23, C12H25, C17H35, C18H37, C21H43, C22H45. 12. Process according to any one of Claims 7 to 11, in which said X group comprises an aliphatic hydrocarbon-based chain having 1 to 2 carbon atoms. 13. Process according to any one of Claims 1 to 12, in which the road bitumen comprises from 0.1% to 5% by weight, preferably from 0.5% to 4% by weight, more preferentially from 0.5% to 2.5% by weight of said chemical additive relative to the total weight of said bitumen. 14. Process according to any one of Claims 1 to 12, in which the road bitumen comprises from 5% to 30% by weight, preferably from 6% to 28% by weight, more preferentially from 7% to 26% by weight of the chemical additive relative to the total weight of said bitumen. 15. Process according to any one of Claims 1 to 14, in which the road bitumen comprises between 0.05% and 15% by weight, preferably between 0.1% and 10% by weight, more preferentially between 0.5% and 6% by weight of an olefinic polymer adjuvant. 16. Use of bitumen blocks as described in any one of Claims 1 to 15 as road binder. 17. Use according to Claim 16, for manufacturing mixes.