Investigate expression of aggregate semantics within a single FHIRPath expression

[johngrimes]

It may be possible to deliver the same functionality as the Aggregate operation (perhaps with less limitations and restrictions) by allowing the user to express the query within a single FHIRPath expression.

This would require us to provide (through functions) a way of describing a set of grouping expressions. It would also require us to specify a "grouped" data type that we can then intelligently deal with and return to the user as a result (i.e. Parameters).

This could be an operation in addition to the current functionality, and perhaps the current operations could be refactored to make use of this as a general-purpose way of deriving them.