Move rho_dh into the density sub-structure. Update the pressure and sound-speed after kick and drift

Moved the structure related to cooling into a separate file. Added a structure to the xpart to contain the accumulated radiative information.

Rearranged and replaced: div_v,curl_v->balsara,entropy_dt,pressure->P_over_rho variables of the part struct.

The particle definition is now split in multiple files. The correct one gets imported depending on the chosen hydro model.

Created an MPI type for the particles. This is useful for particle exchanges when redistributing. Up to 2^31 particles can now be exchanged at once.