Returns
        -------
        List of dicts.  One dict for each document, keys are metadata, values are counts

def list_extra_features(self):
        '''
        Returns
        -------
        List of dicts.  One dict for each document, keys are metadata, values are counts
        '''
        return FeatureLister(self._mX,
                             self._metadata_idx_store,
                             self.get_num_docs()).output()