Allow star formation when running over MPI

This extends the current version of the code by allowing star formation when running over MPI. This means:

• Creating space for the spare particles only for local + foreign cells and not all top-level cells,
• Sorting the spare particles only for local cells,
• Discarding the spare particles when redistributing.
• Fixing #560 (closed).
• Fix a memory leak in the freeing of stars' sort array that follows the creation of a star,
• Move the star formation task to the top-level since it does restructure the spart array in the whole cell,
• Add time-step communication tasks for each particle type rather than one single bundled operation.

The final step (coming soon in a separate branch) is to allow both Star formation and feedback at the same time over MPI.