Get easily a metadata from the CTS namespace

        :param key: CTS property to retrieve
        :param lang: Language in which it should be
        :return: Literal value of the CTS graph property

def get_cts_metadata(self, key: str, lang: str = None) -> Literal:
        """ Get easily a metadata from the CTS namespace

        :param key: CTS property to retrieve
        :param lang: Language in which it should be
        :return: Literal value of the CTS graph property
        """
        return self.metadata.get_single(RDF_NAMESPACES.CTS.term(key), lang)