PROCESS FOR MANUFACTURING ACROLEIN FROM GLYCEROL The present invention relates to the production of acrolein and/or acrylic acid from glycerol and, more particularly, one subject of the present invention is a process for preparing acrolein by dehydration of glycerol in the presence of a catalyst system based on iron phosphorus oxide. Fossil resources, such as oil cuts, for the chemical industry will be exhausted in a few decades. Resources of natural and renewable origin such as alternative raw materials are consequently being studied more. Acrolein, an important synthetic intermediate for the chemical industry is produced industrially by oxidation, in the gas phase, of propylene via the oxygen in the air in the presence of catalyst systems based on mixed oxides. Glycerol, derived from plant oils in the production of biodiesel fuels is one of the routes envisaged as a substitute for propylene, glycerol being able to be subjected to a catalytic dehydration reaction in order to produce acrolein. Such a process makes it possible to thus respond to the concept of green chemistry within a more general context of protecting the environment. Numerous catalyst systems have already been the subject of studies for the dehydration reaction of glycerol to acrolein. A process is known from Patent FR 695 931 for preparing acrolein from glycerol according to which acid salts having at least three acid functional groups or mixtures of these salts are used as catalysts. The preparation of these catalysts consists in impregnating, for example with iron phosphate, pumice that has been reduced to pea-sized fragments. According to the teaching of the patent, the yield obtained with this type of catalyst is greater than 80%. In Patent US 2,558,52.0, the dehydration reaction is carried out in gas/liquid phase in the presence of diatomaceous earths impregnated with phosphoric acid salts, in suspension in an aromatic solvent. A degree of conversion of glycerol to acrolein of 72.3% is obtained under these conditions. Patent US 5,387,720 -describes a process for producing acrolein by dehydration of glycerol, in liquid phase or in gas phase, at a temperature ranging up to 340°C, over acidic solid catalysts that are defined by their Hammett acidity. The catalysts must have a Hammett acidity below +2 and preferably below -3. These catalysts correspond, for example, to natural or synthetic siliceous materials, such as mordenite, montmorillonite and acidic zeolites; supports, such as oxides or siliceous materials, for example alumina (Al2O3), titanium oxide (TiO2), covered by monobasic, dibasic or tribasic inorganic acids; oxides or mixed oxides such as gamma-alumina, ZnO/AI2O3 mixed oxide, or else heteropolyacids. The use of these catalysts would make it possible to solve the problem of formation of secondary products generated with the iron phosphate type catalysts described in the aforementioned document FR 695,931. According to Application WO 06/087084, the strongly acidic solid catalysts whose Hammett acidity H0 is between -9 and -18 have a strong catalytic activity for the dehydration reaction of glycerol to acrolein and are deactivated less quickly. However, the catalysts recommended in the prior art for producing acrolein from glycerol generally lead to the formation of by-products such as hydroxypropanone, propanaldehyde, acetaldehyde, acetone, addition products of acrolein to glycerol, polycondensation products of glycerol, cyclic glycerol ethers, but also phenol and polyaromatic compounds which originate from the formation of coke on the catalyst and therefore from its deactivation. The presence of the by-products in acrolein, especially propanaldehyde, poses numerous problems for the separation of acrolein and requires separation and purification steps which lead to high costs for the recovery of the purified acrolein. Furthermore, when acrolein is used for producing acrylic acid, the propanaldehyde present may be oxidized to propionic acid which is difficult to separate from acrylic acid, especially by distillation. These impurities that are present greatly reduce the field of application of the acrolein produced by dehydration of glycerol. The Applicant Company has therefore sought to improve the production of acrolein from glycerol, by using more selective catalysts that make it possible to obtain high yields of acrolein and that have an activity over long durations. Furthermore, numerous applications are known for catalysts mainly composed of iron phosphate. Among these applications, mention may especially be made of: - FR 1 604 884: Oxidation of olefins and diolefins; - US 3,725,494: Process for producing diolefins; - JP 10-287610: Production of methylglyoxyl by oxidation of hydroxyacetone; - JP 2003-146935: Preparation of pyruvic acid by vapour phase oxidation of lactic acid; - US 3,314,760: Production of chloromethane by oxychlorination of methane; - US 3,173,962: Production of olefins and chlorinated hydrocarbons; - GB 616 260: Polymerization and condensation of olefins; - US 3,142,697: Production of nitriles by ammoxidation of an olefin, and more specifically production of acrylonitrile and methacrylonitrile; - JP 8-295687: Production of citraconic anhydride from lactic acid: - FR 2 499 557: Production of alkyl esters by oxydehydrogenation/esterification of mixtures of carboxylic acids and alcohols; and - US 4,381,411: Oxydehydrogenation of saturated aldehydes to unsaturated aldehydes and more specifically for the production of acrolein and methacrolein. The use of catalyst systems based on iron phosphate for the oxydehydrogenation of saturated carboxylic acids to unsaturated carboxylic acids, in particular the conversion of isobutyric acid to methacrylic acid has been widely described: Mention may be made of Patent Application FR 2,514,756 which describes the oxydehydrogenation reaction of isobutyric acid in the presence of a calcined iron phosphate containing an extrinsic metal such as silver, lanthanum, cobalt or tellurium, as a modifier compound or dopant. In Patent Application FR 2 497 795, the same reaction is carried out in the presence of an iron phosphate modified by the presence of a metal chosen from boron, aluminium, gallium and indium. It has been found that the iron phosphate catalyst modified by aluminium is active for long durations for the oxydehydrogenation reaction of isobutyric acid. Patent Application FR 2 245 604 describes a process for preparing