Add methods to compute edges & faces from elements

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