Sink density tasks

AUTHORS

doc/RTD/source/NewOption/sink_adding_new_scheme.rst

src/cell.c

src/cell_sinks.h

src/cell_unskip.c

src/engine_maketasks.c

src/runner.h

src/runner_doiact_functions_sinks.h

src/runner_doiact_sinks.c

src/runner_ghost.c

src/runner_main.c

src/scheduler.c

src/sink/Default/sink.h

src/sink/Default/sink_iact.h

src/sink/Default/sink_part.h

src/sink/GEAR/sink.h

src/sink/GEAR/sink_iact.h

src/sink/GEAR/sink_part.h

src/space_recycle.c

src/task.c