New unique ID

doc/RTD/source/ImplementationDetails/index.rst

doc/RTD/source/index.rst

examples/main.c

src/Makefile.am

src/cell.c

src/space.c

src/space.h

src/space_unique_id.c

src/space_unique_id.h

src/star_formation/EAGLE/star_formation_struct.h

src/star_formation/GEAR/star_formation_struct.h

src/star_formation/QLA/star_formation_struct.h

src/star_formation/none/star_formation_struct.h