topo: add __getitem__ for Graph

Commit 65c35b65 'Remove networkx dependency' broke this line from RipL:

    nodes = [n for n in self.g[name] if self.layer(n) == layer]

To work around this, RipL code would have to be changed to something
like this:

    nodes = [n for n in self.g.data[name] if self.layer(n) == layer]

...which would use an internal variable, data.

It seems cleaner to add this one little feature from NetworkX Graph objects.