AggregationFn
typealias AggregationFn<TData> = (columnId: String, leafRows: List<Row<TData>>, childRows: List<Row<TData>>) -> Any?
Aggregation function for a grouped column. Receives the column id, the leaf rows of the group and the immediate child rows (or sub-aggregations when grouping deeper than one level), and returns the aggregated value.