curate static retrospective flu hospitalization dataset

[vpnagraj]

up to now, we have used fiphde data retrieval functions to pull hospitalization / ILI information from API sources for all available weeks as part of our forecasting pipeline.

given changes in the NHSN reporting guidelines and transfer to a new API (see #225), it may help to have a pre-curated object with retrospective "NHSN-like" flu hospitalization data at the state and national level (i.e., potentially including time series extension #219 and the non-seasonal gaps filled in #220).

[vpnagraj]

merged into dev via #228