Merge branch 'leaner_code' into 'master'

examples/main.c

src/cell.c

src/cell.h

src/cell_black_holes.h

src/cell_convert_part.c

src/cell_drift.c

src/cell_hydro.h

src/cell_sinks.h

src/cell_stars.h

src/engine.c

src/engine_io.c

src/part.h

src/runner_ghost.c

src/runner_others.c

src/runner_recv.c

src/space_split.c

src/stars_io.h

src/stars_logger.h

src/velociraptor_interface.c

tests/test125cells.c