Context:

While createing distributedBy variable reports, a matchAll filter aggregation had to be added in order to correclty aggregate for undefined/null variables which could be more conveniently handled/more readable if the distributedBy createAggregation methods returned a list - similar how the GroupByVariable returns a list of the variable aggregation and the undefinded/null aggregation. Lets refactor the createAggregation methods to return a list instead.

AT:

• All `createAggregation` methods in our DistributedBys now return a list instead of a single aggregations
• the reverse nested aggregation in ProcessDistributeByVariable has been removed and instead added as a separate entry in the returned list