Better way of dealing with the chemistry i/o. The i/o routines directly call the list construction, lowering the burden on the hydro scheme mainteners.

Corrected typo in Minimal-SPH implementation that prevents neighbour number convergence in some cases.

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

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

Moved the structure related to cooling into a separate file. Added a structure to the xpart to contain the accumulated radiative information.

Replaced all calls to fminf and fmaxf by the macros that was added for the integer min/max functions.