Add functions to obtain isomorphisms between graphs

[vlvrd]

Summary

Add functions to the isomorphism module that return the possible mappings between isomorphic graphs.

Motivation

Currently, Petgraph is only comparing graphs to indicate whether or not graphs are isomorphic. It doesn't allow users to explore the isomorphism solutions, which may be of interest too.

Details

I suggest updating the try_match function from the isomorphism matching private module to return an iterator that contains the isomorphisms between two graphs, then adding public isomorphism functions to return those mappings. This is similar to what NetworkX does in the functions isomorphisms_iter and subgraph_isomorphisms_iter.

I'm going to implement this myself in my fork since I need these mappings anyways, so if there's interest on whether or not these changes should make their way upstream I'll be happy to discuss them here and hopefully send a PR later.