Unfold feedback request

This pull request is aimed at @pboyer for feedback (not for merge) on the initial work generating topology from set of faces or surfaces.  

A few tests are stubbed out and the two that are written generate protogeometry elements and pass.

Extent is a set of classes for wrapping surfaces/faces and edges/curves.  Methods for finding adjacent elements, building a graph of these relationships, and a method for running BFS to generate a potential folding tree.

There is also a method for checking normal consistency of two adjacent faces over some shared edge, and a method for making the two faces coplanar around that shared edge.
I'd suggest creating an interface, IEdge and IFace, that you can subclass with ProtoGeometry specific types.  Then all of your methods that would take and EdgeLikeEntity and FaceLikeEntity would take those types.