Do not activate stars pair tasks and communications for pairs that recurse to zero work.

Because of recursion, pair tasks can reduce to zero work, if the progeny pairs on a lower level have no active particles. Nonetheless, these pair tasks are still activated, along with all the required communications if these pairs span domain boundaries.

This MR adds an additional check (basically a copy of cell_activate_subcell_stars_tasks that does not activate any tasks) that checks whether any active pairs are left after recursion. It then only activates the pair tasks and communications if this is the case. This should reduce the number of active star pair tasks and communications in large volume simulations with a small number of stars per cell.

TODO:

• also update the activation code in cell_unskip_star_tasks