Merge branch 'master' into gadget2-part-update

src/cell.c

src/gravity/Default/gravity.h

src/hydro/Default/hydro.h

src/hydro/Default/hydro_io.h

src/hydro/Default/hydro_part.h

src/hydro/Gadget2/hydro_io.h

src/hydro/Minimal/hydro_io.h

src/io_properties.h

src/parallel_io.c

src/runner.c

src/serial_io.c

src/single_io.c

tests/testSPHStep.c