Separate the time-step information for hydro and gravity tasks into two different set of variables for each cell.

Created a new function to check that a particle is active, without passing in a pointer to the engine.

Replace ti_begin and ti_end by a time bin. Still need to fix cooling(), do_recv() statistics() and propagate types everywhere.

Added debugging checks to the pair and self tasks to make sure cells have been drifted to this point before they get updated.

Check whether particles or cells are active via an inlined function rather than via a direct read of engine->ti_current