covidcast: support querying all counties of a state

[sgratzl]

see also cmu-delphi/www-covidcast#721

a simple workaround would be to support prefix querie: e.g., geo_type = 'county' and geo_value like '06%' would query all counties in California (06).

with the upcoming Python version we could also have a simple mapping that supports the tree navigation from nation -> state -> county.

[krivard]

this is a good idea, though it does require storing a mapping from state codes to their numeric FIPS equivalents. that kind of thing will be much easier to build and test once we're in Flask, but if cmu-delphi/www-covidcast#721 needs the lift earlier than that we might be able to hack something together. There's a workaround using the R or Python clients that doesn't require typing in all the counties in a state by hand; I'll comment in the other thread.

[krivard]

...how do you get from "ca" to "06" then?

[sgratzl]

Duplicate of #542