Merge branch 'mpi_periodic_gravity' into 'master'

examples/EAGLE_12/eagle_12.yml

examples/main.c

src/active.h

src/cell.c

src/cell.h

src/cosmology.c

src/engine.c

src/mesh_gravity.c

src/multipole.h

src/proxy.c

src/proxy.h

src/runner.c

src/runner_doiact_grav.h

src/scheduler.c

src/space.c