consider adding a "us" geo_type to covidcast

[sgratzl]

atm the highest level is state but afaik there is no option to get signal values for the us as a whole. It would be nice to have them since it would provide a nice starting point for a visual drill down operation

[krivard]

Agree; FB asked for this back in July but we weren't in a position to provide it then.

Propose instead the following:

• geo_type: nation
• geo_value: usa

This will need to be handled in each indicator unless we want to get clever with aggregating aggregates. Moving to the indicators repo for further work.

[krivard]

Implemented as

• geo_value: us
  to use standard 2-char country codes.

Remaining work in #199