Implement Head_Func::evaluate_iby()

[st-pasha]

This relates to using an i filter which is an expression, while simultaneously supplying a groupby clause.

There are at least several cases to consider here: one where the expression contains no group columns or reduce operators; another where the filter expression contains only group columns and reduce operators; and lastly the mixed case.

[elliottash]

Hi guys, really fantastic package. In case it helps for priorities, this is a critical feature for making the package usable.

e.g.: df[f.loan_amnt<1000, dt.sum(f.loan_amnt),by(f.grade)]

[samukweku]

Hi Team, any update on this functionality and when it will be implemented?