Retrieve a relation label from the store.

def get(self, cid1, cid2, annotator_id):
        '''Retrieve a relation label from the store.
        '''
        t = (cid1, cid2, annotator_id)
        for k, v in self.kvl.scan(self.TABLE, (t, t)):
            return self._label_from_kvlayer(k, v)