Converts the coreference layer to NAF

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