Also tag all top-level cells with foreign partners, do the multipole exchange after maketasks so that we have all the necessary tags available

Add some additional checks as well

Conflicts:
	src/engine.c

Conflicts:
  src/cell.c
  src/engine.c

Use MPI communicators to reduce the use of tags.

See merge request !591

Put the checks for i/o dumps into a separate function. Call that function after the 0th step as well.

Planetary gravity fixes

Closes #451

See merge request !596

Only record that a top-level multipole has interacted via long range if it actually found any neighbour within range.

Gizmo cosmology

See merge request !594

Conflicts:
	src/space.c

Big update for planetary stuff

See merge request !574

Fix a(t) in cosmo

Closes #450

See merge request !592