Extents don't include the cover of fork points by default. (#998)

This PR removes the requirement that all cover points are included in a region.
The motivation is to allow more flexible definion of regions, particularly the proximal and distal sets thereof.
The other motivation is that the author finds it much simpler to reason about, however others find the existing approach easier to reason about.

The changes:
 * `mextent` does not always include points on the cover at fork points.
   * it still enforces that its constituent cables have no intersections and are ordered.
 * a `super` region expression adds the cover points to a region.
 * update `most_proximal` to return the `minset` of the proximal points in a region's cables.
 * fix some cut and paste errors in comments and printing of locset expressions.