GIZMO hydro is crash free

GIZMO hydro is now stable, in the sense that it should not crash if weird particle configurations arise or masses/densities/pressures/energies become negative. This behaviour can be controlled through a number of new parameters in const.h. With all corrections enabled by default, EAGLE 12 now runs (solving #271 (closed)).

This merge request also contains the changes @matthieu made to make GIZMO work over MPI, and a number of other changes made to the disc patch test (which unfortunately does not work yet). The coupling between gravity and GIZMO is still an issue, but the stability changes can be merged in.