NGM model demo

[swo]

• Showing how to get population-wide $R_0$ and infection distribution from eigenvalue functions in Python
• Confirms that:
  • the next-generation matrix is the entity that one should use
  • the spectral radius is population-wide $R_0$
  • L1-normed eigenvectors are the stable infection distribution

Because this PR was only for demonstration of the math purposes, we don't actually need it in the codebase. This will be replaced by more careful documentation. cc @paigemiller

[paigemiller]

this looks good, but I would like to see a few more examples that cover the package more specifically and also show how to derive the high/low risk group example from the canonical textbook! I think we can merge this in if we don't have time to do this today, and can work on adding to the example next week

[paigemiller]

can we add a few more examples to this demo:

1. the functions in the package NGM, and how they relate to the current example
2. the example from keeling and Rohani? with two groups
3. how to use pop size inputs and vaccine doses

[swo]

Because this PR was only for demonstration of the math purposes, we don't actually need it in the codebase. This will be replaced by more careful documentation. cc @paigemiller