Exposing a previously unused private method that scans an Hbr mesh for
creases and other features and determine the isolation level needed.

Minor improvements:
- we now have a custom "corner" isolation level : with edge-only boundary
  interpolation rules, corner vertices are rounded and would require an
  isolation level of 10. In practice however, 5 appears to be more than
  enough to produce a perfectly rounded shape and is set as a default
  for this value.

- the logic has been made a little more efficient by moving the vertex
  sharpness tags tests to a separate loop so as not to repeat the check
  multiple times for a given vertex

Also added some doxy comments

fixes #161