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.

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.