New the representation of aggregate plans

[hatyo]

This proposes a new representation of aggregate plans that gets rid of the first grouping Value in the underlying select expression.

Doing so would achieve two goals:

• Avoid ambiguous derivation of fields: currently, a grouping field exists twice, once in the grouping value RCV, and another time in the quantifier of the originating table. This is unnecessary and makes integration of the value mapping API more complex.
• Overall simplification in the plan generation logic: maintaining the grouping Value is complex and requires some state that increases the complexity of the plan generator. Getting rid of the grouping Value would help streamlining the logic of the plan generator.