add versioning capabilities for NSSP data

[nickreich]

The changes would happen around here:

iddata/src/iddata/loader.py

Lines 420 to 421 in c597564

	if as_of < datetime.date.fromisoformat("2025-09-17"):
	raise NotImplementedError("Functionality for loading NSSP data with specified as_of date is not implemented.")

Might involve throwing a warning that the data are queried from elsewhere and would be implemented through either

1. Delphi EpiData API
2. truncating the most recent data?

[lshandross]

We've decided to go with option (one from above). Here are some bullet points about how we will implement it:

• Update iddata to fetch older versions of NSSP data from the Epidata portal
  • Simple “fix”:
    • Default to epidata API call, only try load_nssp_from_cdc() if
      • a) loading from epidata API fails
      • b) as_of date is >= 2025-09-13
    • change if clause to load_nssp() to limit based on available dates for data from epidata API, still throwing an error for unsupported as_of dates
    • Add load_nssp_from_epidata_api() to be called that calls the API and transforms data to expected format