map(func) {
        const mapped = [...this].map(({ group }) => group.map(func));
        return this.df.__newInstance__(
            [].concat(...mapped.map(group => group.toCollection())),
            mapped[0].listColumns()
        );
    }
Map on DataFrame groups.
@param {Function} func The function to apply to each row of each group.
@returns {DataFrame} A new DataFrame containing the result.
@example
groupedDF.map((row,i) => row.set('b', row.get('a')*i));