Add covidcast support for geo_type "HSA"

[melange396]

This is motivated by the desire to more properly capture the NSSP dataset, which appears to report county-level information, but it is actually reporting HSA-level information (and repeating the same value for each county in an HSA).

To support HSA as a "geo_type" in the covidcast endpoint, the only absolutely required change would be to add "hsa" to the allowed geographies in the Covidcast CSV importer code:

delphi-epidata/src/acquisition/covidcast/csv_importer.py

Lines 64 to 65 in 274cfe5

	# set of allowed resolutions (aka "geo_type")
	GEOGRAPHIC_RESOLUTIONS = {'county', 'hrr', 'msa', 'dma', 'state', 'hhs', 'nation'}

It should be easy and helpful to also add sanity checks for HSA geo_values during acquisition.

Less easy but potentially even more helpful, we should also add server-side request marshaling/validation for HSA codes, but this needs GeoMapper support; see cmu-delphi/covidcast-indicators#2139 and note the discussion of "NCI Modified" HSA definitions, which are likely to be of consequence to this issue.