hydro_set_internal_energy() and hydro_set_entropy() for GIZMO

In the current version, these two functions don't update the state of the particles. If hydro_get_internal_energy() is called after hydro_set_internal_energy()the returned value is not the value we just set.

The conserved values and the primitives both have to be updated. The idea is that these functions should be used for feedback, cooling, etc. and should set the particles in a new valid state. So the pressure, sound speed, thermal energy etc. all have to be updated such that they are all consistent with each others.

An example of this can be found in the Gadget2 SPH implementation.