Converts the object to NAF (if it is KAF)

def to_naf(self):
        """
        Converts the object to NAF (if it is KAF)
        """
        if self.type == 'KAF':
            self.type = 'NAF'
            for node in self.__get_node_terms():
                node.set('id',node.get('tid'))
                del node.attrib['tid']