MPI communications for the SF split

...smoVolume/SmallCosmoVolume_cooling/small_cosmo_volume.yml

src/cell.c

src/cell.h

src/runner_others.c

src/space.c

src/star_formation/EAGLE/star_formation.h

src/star_formation/GEAR/star_formation.h

src/star_formation/QLA/star_formation.h

src/star_formation/none/star_formation.h