Identify areas for improvement in tutorials and tests

[damonbayer]

Goal

Our tutorials and tests should have realistic examples with good explanations, so that it is easier to verify that code is working as expected.

• Rendered tutorials: https://cdcgov.github.io/multisignal-epi-inference/tutorials/index.html
• Tests: https://github.com/CDCgov/multisignal-epi-inference/tree/main/model/src/test

Some examples:

• when we work with a hospital admissions model, we shouldn't have examples that lead to 0 observed admissions.
• All figures should have titles and axis labels
• Are there any obvious tutorials missing? (Intro to renewal models)

[damonbayer]

This should be a separate PR / code review from #83, but it can be used for inspiration.