Return a column one by one

        :raises: StopIteration

def next(self):
        """Return a column one by one

        :raises: StopIteration
        """
        if self._cur_col >= len(self._rec):
            self._cur_col = 0
            raise StopIteration
        col = self._rec[self._cur_col]
        self._cur_col += 1
        return col