Re-check that a particle has not already been removed by a different thread in the locked section of the code when particles move out of their box.

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

Update the black_holes_struct.h file to contain a structure for the BHs and added functions to manipulate the structure.

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

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.

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.