Obtain a data frame for one COVIDcast signal — covidcast_signal

Obtains data for selected date ranges for all geographic regions of the United States. Available data sources and signals are documented in the COVIDcast signal documentation. Most (but not all) data sources are available at the county level, but the API can also return data aggregated to metropolitan statistical areas, hospital referral regions, or states, as desired, by using the geo_type argument.