Improvements to isomorphism tutorial.

[rossbar]

Minor touchups to the isomorphism notebook.

I've limited the changes in this PR to minor changes in wording, line formatting (i.e. splitting up very long lines, where possible), and minor touchups to links etc.

LMK what you think!

[eriknw]

These changes look good to me. I like using explicit axes and figures in matplotlib instead of relying on pyplot implicit API.

I left one tiny typo suggestion. There were places where I would consider adding a comma, but I don't like being a comma-nazi :-P

[dschult]

In reading through this file, I notice that line 252 should either remove the word "that" or add "are isomorphic" at the end of the line.

Also, on line 58, I think the bijection for the example would be the identity isomorphism. And not f(i) = i+1 as stated.

Perhaps we should pick an example where it is easy (in code) to get differently named nodes. For example, G = nx.wheel_graph(8) and H = nx.wheel_graph(range(1,9)). But maybe the drawings of those networks are not as nice. But the incorrect mapping is confusing as it is.