Automatic diagramming of an odin graph

[richfitz]

• [x] @thibautjombart to work out a nice example and from that determine the data structure (nodes, edges and labels)
• [ ] Rich to work out how to translate the internal graph into that information

[thibautjombart]

Quite keen to do this, and I can't foresee any huge problem except for rendering equations within outputs - not sure if mathjax will work with visNetwork. I will try and get some hours on this next week.

[richfitz]

See #105 for some thoughts on lifting some docs out of the model itself

[richfitz]

I have moved this work onto the i104_model_graph branch