Parsons integration

[chrisdevereux]

Parsons provides an extensive suite of connectors for common tools used by progressive organisations, with a Table abstraction that isn't a million miles away from our Datasource (for list queries, at least).

Investigate a way of allowing Parsons connectors to be used easily in this library.

This might take the form of a Datasource subclass with an abstract method that subclassers implement to return a Parsons table, for example.