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
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.