rationalize API for retrieving flu hospitalization data

[vpnagraj]

once we complete #225 we'll have functionality to retrieve data from NHSN via two different streams. we also have been working on adding another option for pulling FluSurv-Net hospitalization rates, and once we release v2.1.0 get_cdc_hosp() will be able to use either CDC FluView or RESP-NET (see #214). each of these streams has strengths, limitations, and nuances. but there is some common ground on data prep, and the API is getting a little unwieldy. we should consider adding a single "front door" that re-implements the functionality in one place. this would probably be easier for us to document and future us / other users to understand the differences in retrieval methods / sources.

marking this as "fix later" as it will be a major API change, and might lead to deprecation of the current flu hospitalization retrieval methods. so this may even be a v3.X.X release.