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

This reverts commit 8a1c735c.

More uniform condition for the linking of the communication tasks. We need non-zero part in the cell.

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

Also make the code compile with the new function signatures if we are not using the EAGLE black hole model.

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