Function interp_by 

fn interp_by(
    positional: &[RValue],
    named: &[(String, RValue)],
    context: &BuiltinContext<'_>,
) -> Result<RValue, RError>

Apply a function to subsets of a data frame or vector split by a grouping factor.

@param data data frame or vector to split @param INDICES factor or vector defining the groups @param FUN function to apply to each subset @return list of per-group results