Rewrite of MPI loops - Add stellar feedback loops

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!

doc/Doxyfile.in

@@ -775,6 +775,7 @@ INPUT		       += @top_srcdir@/src/chemistry/EAGLE
 INPUT		       += @top_srcdir@/src/entropy_floor/EAGLE
 INPUT		       += @top_srcdir@/src/star_formation/EAGLE
 INPUT		       += @top_srcdir@/src/tracers/EAGLE
+INPUT		       += @top_srcdir@/src/stars/EAGLE
 
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