A query operation that aggregates values by groups. This operation takes a property whose values define groups, and applies a subsequent reduction operation (e.g., Mean, Sum, Max) to aggregate the values within each group. If applied to a vector, the result is a vector with one entry per group. If applied to a matrix, the result is a matrix with one row per group. This is typically followed by a reduction operation that specifies how to aggregate the grouped values. See the Julia documentation for details.