DOSELF_SUBSET, DOPAIR1_NAIVE and DOPAIR2_NAIVE should do their calculation in the frame of the cell.

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

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

Completes d8025de1

Use dx_max_sort_old and not dx_max_sort when checking whether cells have been sorted in DOPAIR1_BRANCH()