Pass the hydro_properties to the prepare_gradient function. Make sure rho_dh is sensible when particles get ejected out of neighbourhoods.

Update the other hydro schemes to also compile with the star formation model and entropy floor model.

Added a function to the hydro scheme to be called when a particle gets removed due to non-periodic BC infringment.

Also updated the Pressure-Entropy SPH scheme to use the new definition of the number of neighbours and its derivative. Back to correct results.

Correctly initialise the signal velocity of the SPH particles in the Minimal and Pressure-Entropy cases.

Initialize the signal velocity to be the particle's sound-speed. Prevents divisions by zero in cases where a particle has 0 neighbours.

Replaced global variables by a new hydro_space struct that is part of space. This struct is then passed on to hydro_init_part, and used to initialize the Voronoi cells. For non SHADOWFAX_SPH schemes, this struct is empty. Replaced random_uniform declarations in some unit tests with the tools.h version. Added some more comments to 3D Voronoi code; it is now almost completely documented (meaning I almost completely understand it now).

Cleaned up Gizmo fixes. Added a hydro_timestep_extra method and added an empty version for all hydro schemes.

Check whether particles or cells are active via an inlined function rather than via a direct read of engine->ti_current