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