Merge branch 'add_particles' into 'master'

README

README.md

doc/RTD/source/CommandLineOptions/index.rst

examples/main.c

examples/parameter_example.yml

src/Makefile.am

src/cell.c

src/cell.h

src/common_io.c

src/engine.c

src/engine.h

src/engine_maketasks.c

src/hydro/Gadget2/hydro_iact.h

src/hydro/Minimal/hydro_iact.h

src/parallel_io.c

src/part.c

src/runner.c

src/serial_io.c

src/single_io.c

src/sourceterms.c