1. Process for the preparation of porous polyolefin particles, which process comprises the following steps: dissolution of at least one crystallizable polyolefin in a solvent, which results in a solution being formed which comprises 0.1-50 wt.% (on the basis of the sum of the weight of the solvent and that of the polyolefin) polyolefin, and the initial polyolefin solution formed containing between 5 ppm and 20 wt.% (on the basis of the sum of the weight of the polyolefin and that of the nucleating agent) of nucleating agent, dispersion of the resulting polyolefin solution in a non-solvent, at a temperature that is higher than the crystallization temperature of the polyolefin in the polyolefin solution, upon which a multiphase system is formed, cooling of the multiphase system, with simultaneous stirring, the cooling rate being between 0.05 and 10 degree C/min, down to a temperature which is below the crystallization temperature of the polyolefin in the polyolefin solution, so that polyolefin-containing particles are formed, separation of the polyolefin-containing particles from the liquid(s), drying of the polyolefin-containing particles at a temperature that is below the crystallization temperature of the polyolefin in the initial polyolefin solution. 2. Process for the preparation of porous polyolefin particles, which process comprises the following steps: dissolution of at least one crystallizable functionalized polyolefin in a solvent, which results in a-solution being formed which comprises 0.1-50 wt.% (on the basis of the sum of the weight of the solvent and that of the functionalized polyolefin) functionalized polyolefin, dispersion of the resulting polyolefin solution in a non-solvent, at a temperature that is higher than the crystallization temperature of the functionalized-polyolefin in the polyolefin solution, upon which a multiphase system is formed, cooling of the multiphase system, with simultaneous stirring, the cooling rate being between 0.05 and 10 degree C/min, down to a temperature which is below the crystallization temperature of the functionalized-polyolefin in the polyolefin solution, so that functionalized-polyolefin-containing particles are formed, separation of the functionalized-polyolefin-containing particles from the liquid(s), drying of the functionalized-polyolefin-containing particles at a temperature that is below the crystallization temperature of the functionalized-polyolefin in the initial polyolefin solution. 3. Process according to either one of Claims 1-3, characterized in that, before the multiphase system is cooled, a portion of the solvent is evaporated from the multiphase system formed. 4. Process according to either one of Claim 1-3, characterized in that the polyolefin is a polyethylene. 5. Process according to Claim 4, characterized in that the solvent is cyclohexane, n-hexane or n-heptane. 6. Process according to any one of Claims 1-5, characterized in that the (functionalized) polyolefin concentration in the polyolefin solution is between 1 and 30 wt.% (on the basis of the sum of the weight of the solvent and that of the polyolefin). 7. Process according to any one of Claims 1 or 3-6, characterized in that the nucleating agent is chosen from the following compounds: silicates, mica, talc, sorbitol derivatives, saturated.or unsaturated fatty acids or their salts, organic pigments, polyolefins with functional groups, or a combination of these. 8. Process according to Claim 7, in which the nucleating agent is talc, a sorbitol derivative or an oxidized polyolefin. 9. Process according to any one of Claims 1-8, characterized in that the non-solvent is water, acetone, methanol, ethanol or a mixture of these compounds. 10. Process according to any one of Claims 1-9, characterized in that the non-solvent contains a surface-active agent. 11. Process according to any one of Claims 1-10, characterized in that the multiphase system is subjected to cooling at a rate that is between 0.1 and 5 degree C/min. 12. Process according to Claim 11, characterized in that the multiphase system is subjected to cooling at a rate that is between 0.2 and 2 degree C/min. 13. Process according to any one of Claims 1-12, characterized in that, upon drying under reduced pressure, the partial pressure of the solvent in the vapour phase is less than 20% of the standard vapour pressure of the solvent at 20 degree C. 14. Process according to Claim 2 and 7, in which the functionalized polyolefin contains from 0.1 to 15 wt.% functional groups. 15. Use of the porous polyolefin particles obtained according to the process of any one of Claims 1-14 as concentrate, absorbent or carrier for substances that are to be released slowly or in a controlled manner.