Lock the space before attempting to remove a particle from the system. This should prevent duplicated removals.

In the recursive unskipping of star sub-tasks, also consider a cell as active when SF is on and the cell is active for hydro.

Revert "Activate star tasks also if we are running with SF on and the cell is active for hydro purposes."

This reverts commit 8a1c735c.

Added communication functions to propagate the new number of star particles after a SF event has occured.

switch to using a 32-bit flag field and atomics, replace '|=' with '= ( ... || ... )', which should do short-circuit evaluation, and is easier to read.

replace c->stars.do_stars_sub_sort, do_drift and do_sub_drift with cell_flag_do_stars_sub_sort, do_grav_drift and do_grav_sub_drift, respectively.

replace c->hydro.do_sub_limiter with cell_flag_do_hydro_sub_limiter. also use uint16_t for the sort masks.