Fix GEAR pressure floor

Big change to the pressure floor API and corresponding changes to the hydro flavours as we need to pass the engine-carried floor throughout. This also has consequences on the cooling API and on the feedback.

At the same time, cleaned up a bit the grackle cooling header.

Also added a few checks to prevent certain feedback modes and hydro models to run with the pressure floor where it is not implemented.

Cleaned up the Agora galaxy example.

Todo:

• Add doxygen strings.
• Apply the same API change to the other hydro schemes.
• Add the higher-res. ICs to the repo. Update run script.