Used to get a claim def from Sovrin
        :param seqNo: reference number of schema
        :param signature: CL is only supported option currently
        :return: req object

def requestClaimDef(self, seqNo, signature, sender):
        """
        Used to get a claim def from Sovrin
        :param seqNo: reference number of schema
        :param signature: CL is only supported option currently
        :return: req object
        """
        operation = { TXN_TYPE: GET_CLAIM_DEF,
                      ORIGIN: sender,
                      REF : seqNo,
                      SIGNATURE_TYPE : signature
                    }

        req = Request(sender, operation=operation)
        return self.prepReq(req)