activate sorts and drifts in sub-cell tasks explicitly, store current values for h_max, dx_max, and dx_max_sort for consistency.

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

remember at which step the cell was last sorted, enforce during drift that no sort is older than its sub-cell sorts.

Added a maximal smoothing length that can be used to prevent particles from becoming too big in vacuum cases near the box edges. Will break the accuracy of the hydrodynamic scheme but is necessary in some situations.