Rename groupedColumns used in rollups/tree tables

[mattrunyon]

We currently call columns that are grouped together for rollups, groupedColumns. This could get confused with columnGroups which will allow for putting columns into a hierarchical group for the UI.

I think we should change groupedColumns to rollupColumns or maybe treeColumns so it is more distinct

[rcaudy]

It might be best to match other aggregation operations. At least in the table API we tend to call these groupByColumns.