Major Description
Spark SQL's new AlgebraicAggregate interface is confusingly named.
AlgebraicAggregate inherits from AggregateFunction2, adds a new set of methods, then effectively bans the use of the inherited methods. This is really confusing. I think that it's an anti-pattern / bad code smell if you end up inheriting and wanting to remove methods inherited from the superclass.
I think that we should re-name this class and should refactor the class hierarchy so that there's a clear distinction between which parts of the code work with imperative aggregate functions vs. expression-based aggregates.
