add concept of edges

[fcollman]

The reliance on trimesh means that you can't accept mesh graphs that have extra edges to facilitate crossing gaps in the mesh faces. If you allowed for this you could support examples where say the PCG had helped you create a mesh that had some extra edges to heal the mesh graph and TEASAR and wavefront would both work I think better on such a thing. Perhaps the radius would get corrupted but it's better than having to heuristically heal the gaps in the mesh which IMO is a very hard problem in some cases.