Support Aggregate ratios against the entire population or subset

From Trello:

"The key here is that we want the numerator to be an arbitrary aggregate over a very specific entity, but have the denominator as the average over a larger group of which the entity is a part.

This is possible via temporary/more complicated from objects, but it's a really common (and helpful!) feature, so it'd be really nice if we could make this easier."

"For collate, this is really the question of 'double aggregations', where we do an aggregation on some index first, and then use those aggregates as inputs onto another function on a different index."