StockFlow exports a Graph() function not extending Catlab's

[KevinDCarlson]

The file src/visualization.jl imports Catlab.Graphics.Graphviz.Graph and then exports Graph, but this Graph does not extend Catlab.Graph so you get ambiguity complaints if you write using Catlab; using StockFlow; Graph. Some notebooks shouldn't/don't run because of this.

[KevinDCarlson]

Relatedly, probably all these great example notebooks should be being run in CI. This breakage probably goes back to Catlab 0.15 starting to export Graph.

[Ong-Yi-Kai]

If anyone needs to run the notebooks, I solved this by replacing Graph(seirh) with StockFlow.Graph(seirh).