Be more conservative when splitting self-task into subs. We don't want to trigger a rebuild straight after.

Modified the splitting rule for self tasks to be less conservative. Added proper documentation of the splitting conditions.

Also create functions to decide when to rebuild and when to split a task. Update the rebuild and split condition to correctly use kernel_gamma.

Add new function to decide on recursion with in a sub_pair task. Makes sure all recursion tests are identical.

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

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

Added function to drift the multipole of a given cell without updating its progenies.