Moved that functionality into `abp.GraphState`, so that you can
just run `state = abp.GraphState(nx.Graph(((0,1),)))`, `state =
abp.fancy.GraphState(nx.Graph(((0,1),))` or whatever.

Now you can write `abp.fancy.GraphState(nx.Graph())`.
TODO: make this work for a non-fancy `GraphState`

Trying to get more sensible defaults for people who like to think about
graphstates rather than circuits.

Repsponse to issue #4: [Link](https://github.com/peteshadbolt/abp/issues/4)