This includes and supersedes @lhausammann's !700 (closed) merge request.
Also fixes #522 (closed), #476 (closed), #449 (closed) the original issue of #515 (closed) (the negative wait, not the hanging), possibly #537 (closed) and #520 (closed).
The main changes are:
- Each bloc (hydro, gravity, stars) of tasks is now at a fixed level. Only local tasks can move up and down between levels,
- MPI communications happen within one bloc only,
- Add MPI communications for stars density and feedback (original !700 (closed)),
- Add a separate drift task for the stars.
- Move all the star feedback tasks to after the star formation task (itself after the cooling).
This was heavily tested but since it's a big change, a third pair of eyes is very much welcomed!