Conflicts:
	src/runner.c

Foreign cells will need MPI tasks enabled if active

change space_getcell to space_getcells, can pick up more than one cell at a time.

As discussed in #255. Have only checked that it works, haven't done any benchmarks, but this can only make things faster 😉 

See merge request !300

Call vector

Call vectorised version of `runner_doself1_density` when vectorisation is turned on.

See merge request !302