Subset sorting

Massive change in the way we treat sub-cell tasks and sorts in general, i.e.:

 * Sorts and drifts now unlock *all* sorts and drifts above them, this ensures they are executed bottom-up even if intermediate tasks are not active,
 * Sub-cell task hierarchies are traversed consistently during density/ghost/force computations,
 * Sub-cell tasks no longer do the drifting and sorting themselves,
 * Sorts are only re-computed once the oldest sorted index goes stale.
 * Ghosts exist at a lower level in the tree.

See merge request !343