Add methods to compute edges & faces from elements

[goxberry]

Add some utility methods to compute edge & face information from
subclasses of GmshElementBase.

Add:
- gmsh_edge_tuples method to all subclasses of GmshElementBase;
computes a tuple representation of all edges in an element
- gmsh_face_tuples method to all subclasses of GmshElementBase;
computes a tuple representation of all faces in an element
- get_gmsh_edge_indices method to GmshElementBase; maps tuple
representation of edges to node index representation of edges
- get_gmsh_face_indices method to GmshElementBase; maps tuple
representation of edges to node index representation of edges

[goxberry]

I ended up writing these utility functions as part of developing a subclass of GmshMeshReceiverBase that inserts cohesive elements into cracked Gmsh meshes. If an example of that would be interesting, I could contribute an implementation I'm currently working on.

[inducer]

Thanks for taking the time to put together this pull request. I ultimately don't think that people will wind up using the meshpy classes as their internal mesh representation (or, at least, that's not their intended purpose). As such, I'm reluctant to add this code, which is a little separate from the purpose of the code surrounding it.

I agree that it's annoying that essentially every user will wind up writing a version of what you've written (I have!), but I don't know a really good solution to this problem...

Andreas