Removed GIZMO scheme specific hydro_g/setters.h and moved their contents to hydro_flux.h. Moved the centroid calculation to hydro_velocities.h and removed centroid variables from MFM particle.

Streamlined names of hydro variables in GIZMO schemes. Removed almost all overlap between MFM and MFV. Still need to get rid of some ifdefs in hydro.h.

Refactoring GIZMO schemes to avoid code duplication, step 1: moved MFM and MFV to subdirectories of new Gizmo directory, made common version of hydro_*.h included from rest of code that (for now) simply calls the right version.

Compile the gravity code as a separate convenience library. Use special compilation flags for this library. When compiling on AVX512, add extra flags to compile the gravity files.

Moved the particle-carried FOF fields to a separate structure. Write the groupID as a long long and not an int.

Fixes: 548