GEAR: add star formation

examples/parameter_example.yml

src/cell.c

src/collectgroup.c

src/cooling/grackle/cooling.h

src/engine.c

src/engine.h

src/hydro/Gadget2/hydro_part.h

src/runner.c

src/runner_doiact.h

src/space.c

src/star_formation.c

src/star_formation/EAGLE/star_formation.h

src/star_formation/EAGLE/star_formation_logger.h

src/star_formation/EAGLE/star_formation_logger_struct.h

src/star_formation/EAGLE/star_formation_struct.h

src/star_formation/GEAR/star_formation.h

src/star_formation/GEAR/star_formation_iact.h

src/star_formation/GEAR/star_formation_io.h

src/star_formation/GEAR/star_formation_logger.h

src/star_formation/GEAR/star_formation_logger_struct.h