Hierarchical topic model visualization #47
Description
The graphviz functions for visualizing the topic model are finicky and it's costly to have to update them with respect to both networkx and graphviz.
Two proposed options :
1 . We add a function that makes it easy to get to the hierarchy edge list so that others could more easily visualize the hierarchy themselves
2. We go a step further and rework the visualization code so it just uses networkx