- updating filament for external filament incident with >1 region

See the graph at the end of the notebook for the case to handle.

The filament (2,1) is shared by the two faces (4,3,5,2,4) and (5,2,6,5). Adding it as external to one of these will result in a failure when walking around the node or a face.

Prior to inserting the filaments, we have the faces. So the alternative approach would be

 1. Find the incident node for the filament (2) 
 2. Enumerate the links ccw around the node 2: (2,6), (2,5), (2,4)
 3. Determine which pair of these links is the link (2,1) between (2,4), (2,6)
 4. Insert and update the pointers
![screen shot 2013-07-24 at 9 15 28 am](https://f.cloud.github.com/assets/118042/849883/511e294e-f47c-11e2-84be-b0f3e6b23fc7.png)