Do not clear the sort flag when receiving parts after the ghost. We can re-use the sort from the density.

Don't lock the tree for the grav_init task. Don't call the unsorted pairs at all. Skip the entire zeroing loop when no xparts in runner_do_sort()

Moved the sorting entry structure to a separate file following @jwillis' work in the vectorisation branch.

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).

Simplify notation by passing only the field tensor rather than the whole multipole structure to the gravity functions.

Added a maximal smoothing length that can be used to prevent particles from becoming too big in vacuum cases near the box edges. Will break the accuracy of the hydrodynamic scheme but is necessary in some situations.