GEAR: Make Gizmo MFV and MFM compatible with sinks and SF

I made GEAR SF and sinks compatible with GIZMO MFV and MFM. Now we can run simulations with MFM/V as the hydro solver and GEAR physics.

I made some tests with the Sink homogeneous box example with both grackle_0 and grackle_3. I include them here for future reference and as proof that it "works".

Illustration

Note that the simulations do not end at the same time because of:

space_regrid.c:space_regrid():141: Must have at least 3 cells in each spatial dimension when periodicity is switched on.
This error is often caused by any of the followings:
 - too few particles to generate a sensible grid,
 - the initial value of 'Scheduler:max_top_level_cells' is too small,
 - the (minimal) time-step is too large leading to particles with predicted smoothing lengths too large for the box size,
 - particles with velocities so large that they move by more than two box sizes per time-step.

Sometimes I stopped them by hand because Gozmo was taking ages to run: the timesteps are smaller than SPHENIX.

Grackle_0 (equilibrium cooling)

SPHENIX density projection movie MFM density projection movie MFV density projection movie

SPHENIX Phase space diagram MFM Phase space diagram MFV Phase space diagram

Grackle_3 (non-equilibrium cooling with 12 species network)

SPHENIX density projection movie MFM density projection movie MFV density projection movie

SPHENIX Phase space diagram MFM Phase space diagram MFV Phase space diagram