Removed the 'tight' property of the tasks. Only density pairs need to be tight so we can just use that.

Add a separate timer for subset sub-cell tasks. Write the timers to a file. Make sure we can compile without timers. Add a configure option to switch the timers on.

Rebuild criteria

Instead of rebuilding whenever the cell constraints or sort indices are violated, just re-sort in case of the latter.

This lead to some other changes:

 * Init tasks are gone, particle initialization is now handled in the drift tasks,
 * Drift tasks no longer act on the super-cell, but exist at every level similar to the sort tasks,
 * Fixed several dependency issues with sort and drift tasks,
 * Only drift/sort on-demand in the sub-cell tasks.

Marked as [WIP] as this has not yet been tested for MPI runs, or with gravity.

See merge request !327

Gravity multi dt



See merge request !333

Branch to a different, more accurate, calculation in the long-range that don't match the MAC any more.

Don't lock the tree for the grav_init task. Don't call the unsorted pairs at all. Skip the entire zeroing loop when no xparts in runner_do_sort()