Add titles to visualizations

Currently, our GraphViz visualizations consist solely of diagrams of our physics alone. This is fine for REPL-driven model development, where the context is clear from the “EP” part of REPL.

However, for presentation and recording physics experiments, the interactive context in which a physics was created is lost.

So, we should document how to add titles to `to_graphviz` output, and provide helper functions

Future PRs can consider other plotting elements. We can also consider adding legends to our plots, following up on work done in PR #90 which colors components via the physics they came from.

This could be considered a step towards a more “literate” programming style, and a small step towards self-documenting physics experiments. At the extreme, we could imagine compiling a Decapodes script to a pdf documenting the physics, their composition, initial data, and plots.