Better graphviz support for bipartite graphs

Hi @epatters, following up here from the zulip conversation. Before working on `PropertyBipartiteGraph` and `to_graphviz` for it, I wanted to ask if you can help explain the design considerations behind the existing `PropertyGraph` machinery and its interaction with the other graph acsets/`to_graphviz` system, so that I can replicate as much as possible those designs for the bipartite one. And if there are any essential desiderata for the bipartite one. Thanks!