Internal energy and entropy getters for Gizmo

The functions returning the entropy and internal energy for the hydro schemes have slightly changed when introducing more advanced cooling models. We now have a function giving the entropy (physical or co-moving) at the last kick step hydro_get_physical_internal_energy() and hydro_get_comoving_internal_energy() as well as functions returning the values drifted to the current time hydro_get_drifted_physical_internal_energy() and hydro_get_drifted_comoving_internal_energy().

I have written simple versions of these in the MFV and MFM cases. They compile and work but may not be the best choices of content. Specifically, they both do the same, which should probably not be the case.

It would be good to cross-check this and if needed improve on them.