CombineFirst class/method

[jc-harrison]

We would like to be able to combine the outputs of two subscriber location queries, such that a subscriber's location is assigned from the first query if they appear in that query result, or the second query otherwise (i.e. SELECT subscriber, COALESCE(location_from_first_query, location_from_second_query)).

I'd suggest a useful way to implement this would be to add a new query class that takes two queries, joins on specified join_columns, and coalesces values in specified combine_columns. This is somewhat similar to the pandas.DataFrame.combine_first method, and I propose to name it accordingly.

We could also add a Query.combine_first method to wrap this class (along the lines of the existing join, subset, union methods).