Export named graphs from Catlab.Graphs

[epatters]

CC @slwu89

[slwu89]

Export all the things! Thanks. After you merge this I'll update my #850 before requesting review.

[epatters]

Now that vertex_name and edge_name are part of the official API, we should update GraphvizGraphs and GraphCategories to use those by default, which will work for named graphs while preserving the old behavior for unnamed graphs.

[slwu89]

Ok, happy to do that, as it'll help figure out how to adjust #850.

[slwu89]

@epatters I'm actually not sure what's the best way forward on updating GraphvizGraphs to use vertex_name and edge_name. Currently there's a lot of flexibility, as the argument node_labels can be boolean or a symbol referring to a subpart. For example, if we pass a NamedGraph to to_graphviz we have the option of printing the vertex ID, a subpart associated to vertices, or nothing. It would seem that using vertex_name in place of the method node_label for example, would mean we are forced to use the vname subpart.