sink : improved implementattion

doc/RTD/source/Task/current_dependencies.rst

doc/RTD/source/Task/sink.dot

doc/RTD/source/Task/sink.png

src/Makefile.am

src/cell.c

src/cell_drift.c

src/cell_sinks.h

src/cell_unskip.c

src/chemistry/GEAR/chemistry.h

src/chemistry/GEAR/chemistry_struct.h

src/engine.c

src/engine_maketasks.c

src/engine_marktasks.c

src/engine_split_particles.c

src/hydro/AnarchyPU/hydro_part.h

src/hydro/Gadget2/hydro_part.h

src/hydro/Gasoline/hydro_part.h

src/hydro/Gizmo/MFM/hydro_part.h

src/hydro/Gizmo/MFV/hydro_part.h

src/hydro/Minimal/hydro_part.h