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.

mentioned in merge request !816 (closed)

Should that remain open? It was never merges as it was labelled as WIP but I guess this is now too out of date?

It would be good to have a copy of this code around, since I still hope to return to this one day. I guess the branch will still exist, but having the MR around makes it easier to find.

Ok, then we can keep it as it is.