Added instantaneous accretion rate to the EAGLE BH snapshots. Compute the time-step size from the cosmology model for BH accretion.

Collapse send recv tasks

See merge request !790

Conflicts:
	src/cell.c

Star formation sub grid update

See merge request !803

No cooling limit

See merge request !798

Make plots nicer in plotsolution and add temperature histogram, also change function call in star_formation.h to get the entropy floor

Cell flags

See merge request !792

Conflicts:
	src/cell.c

Mark the flags `volatile`, just to be safe. This is not a problem now, but I would hate it to become one later.

Subcell recursion

See merge request !788

Do not force the star particle to be a constant in the default feedback model. This is used for debugging.