Cleaned up GIZMO gravity a bit. Still doesn't work. Added a gradient interpolation limiter that should solve the Noh problem.

Streamlined GIZMO_FIX_PARTICLES usage (tried to add parameter, but this does not work, as space_init has no access to the hydro_properties). Moved conserved variable initialization to hydro_first_init_part, and removed it from hydro_convert_quantities. Added some sanity checks to maintain positive density and pressure.

Add debugging checks to make sure all the active tasks are scheduled and that no inactive task is being run.

Promoted the external gravity tasks to a self sub-type such that the super pointer can be set correctly.

Added space struct as an argument to potential_init. Can use to set centre of potential to be the box centre

Move rho_dh into the density sub-structure. Update the pressure and sound-speed after kick and drift

Re-factored the whole external potential mechanism to match what is done for cooling. Renamed 'disk' to 'disc'.