Merge branch 'restart-structs' into 'master'

examples/main.c

examples/parameter_example.yml

src/Makefile.am

src/clocks.c

src/clocks.h

src/cooling.c

src/cooling.h

src/engine.c

src/engine.h

src/gravity_properties.c

src/gravity_properties.h

src/hydro_properties.c

src/hydro_properties.h

src/parallel_io.c

src/parser.c

src/parser.h

src/partition.c

src/partition.h

src/physical_constants.c

src/physical_constants.h