mimic rhino's network surface with the *vast* difference of tangential continuity

yep, >>> tangential continuity >>> you heared it

see discussion on the pythonocc list:

https://groups.google.com/forum/#!topic/pythonocc/4DkO1R67m0k

and Roman Lygin's blog:

http://opencascade.blogspot.dk/2010/03/surface-modeling-part6.html