start cleanup star_density

src/active.h

src/cell.c

src/cell.h

src/engine.c

src/part.h

src/runner.c

src/runner_doiact_stars.h

src/stars.h

src/stars/Default/stars.h

src/stars/Default/stars_debug.h

src/stars/Default/stars_iact.h

src/stars/Default/stars_io.h

src/stars/Default/stars_part.h

src/stars_io.h