Converts the coreference layer to KAF

def to_kaf(self):
        """
        Converts the coreference layer to KAF
        """
        if self.type == 'NAF':
            for node_coref in self.__get_corefs_nodes():
                node_coref.set('coid',node_coref.get('id'))
                del node_coref.attrib['id']