paisible-wanderer

@vanitu I belive you should use [DataFrame#aggregate](https://www.rubydoc.info/gems/daru/Daru/DataFrame#aggregate-instance_method) ``` df.group_by(:a).aggregate( avg_d: ->(df) { df[:d].mean }, sum_c: ->(df) { df[:c].sum }, avg_of_c: ->(df) { df[:c].mean }, size_b_with_lambda: ->(grouped){ grouped[:b].size}, uniq_b_with_proc: proc {|grouped|...