Updated the default values of space_subsize_pair space_subsize_self to the optimal values found in a parameter search using EAGLE_25 on 16 cosma-5 cores.

Removed the now unused space_maxcount global variable. Documented the new YAML parameters in the example parameter file.

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).

Compute the total mass in the system and add a variable to the gpart to record how much mass they interacted with via self-gravity.

Added the option to replicate the particles along the axis to create artificially larger test cases. Driven by the parameter 'InitialConditions:replicate'.

one big messy commit for the status quo, forces sorts of the active cells in each step, does not use task_type_sub_*, and probably sorts some cells twice. but it works.

Added an (expensive) debugging check to verify that all cells have been drifted to the current point in time before a rebuild or a repartition.