Multilevel model tutorial with dynamical systems

[SamWitty]

Subsumes #351 .

TODO:

• Render graphical model
• Add basic intervention on stratum without observations
• Clean up figures
• Add explainer text throughout

[djinnome]

Why is sir_observation_model a recursive function?

Edit: Nevermind. I now see that single_observation_model calls sir_observation_model with one less dimension.

[djinnome]

In bayesian_multilevel_sir_prior the variable called plate should be called strata