Remove default value for select io

src/gravity/Default/gravity_io.h

src/hydro/Default/hydro_io.h

src/hydro/Gadget2/hydro_io.h

src/hydro/GizmoMFM/hydro_io.h

src/hydro/Minimal/hydro_io.h

src/hydro/PressureEntropy/hydro_io.h

src/hydro/Shadowswift/hydro_io.h

src/io_properties.h

src/parallel_io.c

src/serial_io.c

src/single_io.c

src/stars/Default/star_io.h