Merge branch 'subset_sorting' into 'master'
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