Aggregate transformation works only with FieldPositionKeys

[fhueske]

In the new Java API, Aggregate transformations can only be applied to DataSets that are grouped using field positions and to ungrouped DataSets.
DataSets grouped with KeySelector functions are not supported.

Since Aggregations only work on Tuple DataSets which can be grouped using the more convenient field positions, this might be OK.
Or should we support KeySelector groupings as well for completeness?

Opinions?

[markus-h]

In my opinion it is enough to support aggregrations with tuple fields and field positions only. I cannot think of an usecase where key selectors on tuples are more convenient.

[StephanEwen]

It would be nice, but I think this has rather low priority. We should give a good error message, though.

[fhueske]

I added an error msg in #717