Implement approximate regional feature assignment based on region overlap

[dickscheid]

Currently, regional data features are only found if the parcellation matches the associated region specification. It would be good to allow approximate links, so that features are also found by significatn region overlaps even if the region specification does not match the given parcellation.

Testing all possible region overlaps is computationally intensive, so this will require pre-computation of overlaps between support parcellations.