Implement sinks

doc/Doxyfile.in

examples/IsolatedGalaxy/IsolatedGalaxy_sink/getIC.sh

...es/IsolatedGalaxy/IsolatedGalaxy_sink/isolated_galaxy.yml

examples/IsolatedGalaxy/IsolatedGalaxy_sink/makeIC.py

examples/IsolatedGalaxy/IsolatedGalaxy_sink/run.sh

examples/main.c

examples/main_fof.c

src/cell.c

src/cell.h

src/common_io.c

src/common_io.h

src/distributed_io.c

src/engine.c

src/engine.h

src/engine_drift.c

src/engine_redistribute.c

src/engine_split_particles.c

src/gravity/Default/gravity.h

src/gravity/MultiSoftening/gravity.h

src/io_properties.h