Stars: Feedback and density work over MPI

src/cell.c

src/cell.h

src/engine_maketasks.c

src/engine_marktasks.c

src/error.h

src/runner.c

src/runner_doiact_stars.h

src/scheduler.c

src/scheduler.h

src/space.c

src/stars/Default/stars.h

src/stars/Default/stars_iact.h

src/stars/Default/stars_io.h

src/stars/Default/stars_part.h

src/task.c

src/task.h