return metadata for ALL location / variable combos that don't have data?

[dschristianson]

Consider making a metadata entry for every location / variable combination specified?

Issue #19: We handled empty data returns from the datasource by making a metadata entry for that return with no start / end dates or number of records. We anticipate this to be useful for users to understand what locations - variables combos do not have data.

For USGS, empty data value arrays are returned for location - variable combos that exist in the USGS dataset but that do not have values for the date range specified. Unclear how other data sources might handle this.

If we expand the metadata return to include ALL location-variable combos regardless if the data source returns an empty array or not, it would probably be good to indicate which case it is (we generate an empty combo versus the data source returns an empty data array with what that means for each data source).