WIP GIZMO entropy switch
Reimplementation of !816 (closed) on top of the refactored version of Gizmo. !816 (closed) can now be safely closed.
All relevant code from the original merge request has been reimplemented. I still need to run a complete set of hydro tests to make sure this does not break anything. I have not tested yet whether this fixes the issues with the Zeldovich pancake.