Get feature names (column labels).

        Returns
        -------
        feature_names : list or None

def feature_names(self):
        """Get feature names (column labels).

        Returns
        -------
        feature_names : list or None
        """
        if self._feature_names is None:
            self._feature_names = ['f{0}'.format(i) for i in range(self.num_col())]
        return self._feature_names