Genericize hospital admissions to delayed_incidence_observations

[damonbayer]

There is nothing special about the hospital admissions concept. In general, there are many cases where you would want to take some history of latent (or observed) infections/cases/hospitalizations and observe some proportion of them with some delay: E.g. latent infections -> observed cases -> observed hospital admissions -> observed icu admissions -> observed deaths.

More broadly, I would like remove the hospitalization model as a module, but leave in place the tools to create a hospitalization model, which would be demonstrated in a tutorial.

[damonbayer]

Closed in #398