represent an aggregation function using a set of aggregation keys rather than a KeyExpression

[normen662]

Currently the class IndexAggregateFunction (and derivates) use a KeyExpression called operand to represent both grouping expressions, and grouped expressions (the aggregate function's argument(s)). We also allow for certain attributes of this class, specifically index to be nulled out or not. In general it appears as if there are two different versions of objects created from this class:

1. function objects that do not have an index (they are unbound)
2. function objects that do have an index set (they are bound to that said index)

Interestingly, there is no point fixing the order of the grouping expressions when such a function is not yet bound to an index as the index dictates the order of these expressions.

This issue should target the separation of these two use cases into two actual classes, one for the bound and one for the unbound case. In the unbound case we want to represent the grouping keys in set rather than positionally fixed.