Version that correctly flags the BHs to swallow and removes them before dying in the time-step calculation function.

Split the star formation task into two parts: The star formation bit and the resorting bit. Move the sf_count communication to just depend on the first one in order to speed things up.

entry is a very generic name and it clashes with a struct in the search.h header

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.

Mark the flags `volatile`, just to be safe. This is not a problem now, but I would hate it to become one later.

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

Don't pass enums in the `cell_*_flag` functions, but rather use the type of the `flags` value itself. Using the enum causes problems when passing the union of more than one flag.

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.