A function that returns True iff the edge attribute dictionary for
the pair of nodes (u1, v1) in G1 and (u2, v2) in G2 should be
considered equal during the isomorphism test. The function will be
called like:
edge_match(G1[u1][v1], G2[u2][v2])
That is, the function will receive the edge attribute dictionaries
of the edges under consideration. If None, then no attributes are
considered when testing for an isomorphism.