drift on demand working

examples/SedovBlast_3D/sedov.yml

src/active.h

src/cell.c

src/engine.c

src/hydro/Gadget2/hydro.h

src/hydro/Gadget2/hydro_part.h

src/runner.c

src/runner_doiact.h

src/scheduler.c

src/space.c

src/space.h

src/task.c