API doc for new REST endpoint: /stat/collection

[beets]

This endpoint allows a user to get the value associated with a statistical variable for a set of child places of a certain type, for a given date.

Endpoint: /stat/collection
Parameters:

• parent_place
• child_type
• date
• stat_vars (list)

Available as GET

Example:
https://api.datacommons.org/stat/collection?parent_place=country/USA&child_type=State&date=2013&stat_vars=Count_Person

Note, a direct parent/child place relationship is required here as well, i.e. country/USA -- State works, but country/USA -- County would not:
https://api.datacommons.org/stat/collection?parent_place=geoId/06&child_type=County&date=2013&stat_vars=Count_Person
https://api.datacommons.org/stat/collection?parent_place=country/USA&child_type=County&date=2013&stat_vars=Count_Person