Add a "group by" clause to the query

        :param columns: The columns to group by
        :type columns: tuple

        :return: The current QueryBuilder instance
        :rtype: QueryBuilder

def group_by(self, *columns):
        """
        Add a "group by" clause to the query

        :param columns: The columns to group by
        :type columns: tuple

        :return: The current QueryBuilder instance
        :rtype: QueryBuilder
        """
        for column in columns:
            self.groups.append(column)

        return self