Slimming down of foreign gpart + reduced comm size

src/active.h

src/cell.c

src/cell.h

src/cell_grav.h

src/cell_pack.c

src/cell_unskip.c

src/engine.c

src/engine_fof.c

src/engine_maketasks.c

src/fof.c

src/gravity/Default/gravity.h

src/gravity/Default/gravity_part.h

src/gravity/MultiSoftening/gravity.h

src/gravity/MultiSoftening/gravity_part.h

src/gravity_cache.h

src/part.c

src/part.h

src/runner.h

src/runner_doiact_grav.c

src/runner_main.c