Added description and scale-factor dependence to all the EAGLE i/o fields

src/black_holes/EAGLE/black_holes_io.h

src/black_holes/EAGLE/black_holes_part.h

src/chemistry/EAGLE/chemistry_io.h

src/chemistry/GEAR/chemistry_io.h

src/common_io.c

src/cooling/EAGLE/cooling_io.h

src/cooling/grackle/cooling_io.h

src/gravity/Default/gravity_io.h

src/hydro/Minimal/hydro_io.h

src/parallel_io.c

src/serial_io.c

src/single_io.c

src/star_formation/EAGLE/star_formation_io.h

src/stars/EAGLE/stars_io.h

src/tracers/EAGLE/tracers_io.h