Posterior mean
        $$
        K_{xx}v
        v := \texttt{Woodbury vector}
        $$

def mean(self):
        """
        Posterior mean
        $$
        K_{xx}v
        v := \texttt{Woodbury vector}
        $$
        """
        if self._mean is None:
            self._mean = np.dot(self._K, self.woodbury_vector)
        return self._mean