Merge branch 'rebuild_criteria' into 'master'
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