Gravity only tests

Reverted to old space_recycle and space_regrid versions and bug fix in space_rebuild to use nr_gparts instead of nr_parts when extracting cell counts from sorted indices

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.

Don't pass the cell_min as an argument to space_rebuild(). Just use the value stored in the space structure.