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.
 You might leave out the triple slash comments until you've finalized the design.