Define graph isomorphism methods

[arashbm]

Currently the relabel_nodes test only checks number of nodes and edges. It should check that the two graphs are isomorphic, but the hypothetical dag::is_isomorphic function doesn't exist yet.

Maybe worth checking out automorphism algorithms at the same time.

[arashbm]

So far I've found these promising options:

• dejavu
• Bliss
• nauty and Traces

Rejected:

• Saucy3 (Incompatible license)