return predicted MDEV of noise-type at given tau

def mdev(self, tau0, tau):
        """ return predicted MDEV of noise-type at given tau

        """
        prefactor = self.mdev_from_qd(tau0=tau0, tau=tau)
        c = self.c_mvar()
        mvar = pow(prefactor, 2)*pow(tau, c)
        return np.sqrt(mvar)