Geo Mapper: Return all contained/child regions

[krivard]

Given a geo id (e.g. "ca" for California, a state) and an enclosed geo type (e.g. "county"), return a set of geo ids of the specified type that lie within the specified geo id (e.g. all counties within California)

We don't need to support all possible pairs. For now, we just need:

• all states within a nation
• all counties within a state
• all states within an hhs region

We should not support the following pairs, because they do not nest neatly (some regions overlap more than one region of the larger geo type):

• all msas within a state
• all zips within a county
• possibly others, extend this list as you find them

Related to: cmu-delphi/delphi-epidata#542