add methods to exporter for accessing halfedges for vertices and faces and for accessing next halfface, incident vertex and incident face for halfedges

refs #57

Update documentation of split_copy to reflect the fact that properties are copied from the old face to the new ones.

TriConnectivity: Added two functions split_edge and split_edge_copy to mask the PolyConnectivity functions of the same name (Prevents creation of valence 2 vertices on trimeshes)

closes #51

Updated changelog