Use the same naming convention for the hydro sort activation function and the star sort activation function.

Use the CoM at rebuild time and not the actual CoM in cell_can_use_pair_mm_rebuild() (as the documentation actually suggests....)

Create the M-M gravity task one level higher. The task itself recurses on its progenies. We can run with a rebuild every time-step. More work required on the unskip.

Fixes and documentation of a few more places where the implementation will need to be updated in the future.