Make infection models composable and more modular

[seabbs]

Currently infection models don't match the composability of the rest of the tooling. In particular you cannot compose multiple iterative processes together. It seems like we should do this. One choice we should make is how much we want to support this across infection models and how much we want to focus on the renewal based generative process.

I think we should support all equally but this probably means implementing slightly less efficient iterative direct infections and growth rate models (and for these models several of the wrapper models we might use might not make sense to use).