Updated the way we converge towards the smoothing length. We now use a Newton-Raphson scheme. This applies to the Gadget-2 scheme.

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.

Updated the kick operation to the new framework. Need to do the preparation for the prediction next.

Replaced all calls to VEC_RECIPROCAL and VEC_RECIPROCAL_SQRT with vec_reciprocal and vec_reciprocal_sqrt.