Merge branch 'distributed_io' into 'master'

doc/RTD/source/ParameterFiles/output_selection.rst

doc/RTD/source/ParameterFiles/parameter_description.rst

doc/RTD/source/Snapshots/index.rst

examples/EAGLE_low_z/EAGLE_6/eagle_6.yml

examples/parameter_example.yml

src/Makefile.am

src/black_holes_io.h

src/common_io.c

src/common_io.h

src/distributed_io.c

src/distributed_io.h

src/engine.c

src/engine.h

src/parallel_io.c

src/parallel_io.h

src/serial_io.c

src/serial_io.h

src/single_io.c

src/single_io.h

src/xmf.c