Merge branch 'master' into timestep_order

examples/main.c

examples/parameter_example.yml

src/Makefile.am

src/engine.c

src/engine.h

src/gravity_properties.c

src/gravity_properties.h

src/hydro_properties.h

src/parallel_io.c

src/serial_io.c

src/single_io.c

src/swift.h