add a new engine_step_prop_stop which makes the runners quit if set. engine_clean now sets the prop, releases the runners, and pthread_waits on them.

General style improvement and documentation of the core FOF routines. Do not pass the 'space' structure into the routines to avoid indirections.

Fixed the debugging checks in runner_do_recv_spart() to also be valid for the case with star formation.

Changed the early abort rule in the runner_do_unskip_stars() function to include the case of a run with star formation.

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.

Added instantaneous accretion rate to the EAGLE BH snapshots. Compute the time-step size from the cosmology model for BH accretion.

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.