Expose RadiusOfGyration via api

[greenape]

Pending #600

• Add marshmallow spec for RadiusOfGyration
• Add to allowable joined aggregate queries
• Add flowclient function radius_of_gyration(start, stop, subscriber_subset)
• Add test to integration_tests/tests/test_queries.py
• Add docs/source/4-developer.md with newly added query
• Change returned column name to value from rog in RadiusOfGyration in SQL and column_names attribute
• Update tests using RadiusOfGyration to reflect changed column name

[OwlHute]

Implemented all required code changes, and down to one test error:

Aggregation unit must be specified when running a query.. API returned with status code: 400.{'msg': 'Aggregation unit must be specified when running a query.'}

So it looks like the three parameters given above (start, stop, subscriber_subset) should be augmented by something like aggregation_unit ?!

[greenape]

radius_of_gyration needs to be used in conjunction with joined_spatial_aggregate, because it has no inherent spatial unit attached.

[OwlHute]

I'm currently testing a couple of small changes which may fix the remaining errror. So please hold off on review for now. I'll let you know when the changes have been pushed ..