Computes the posterior covariance between points.

        :param X1: some input observations
        :param X2: other input observations

def posterior_covariance_between_points(self, X1, X2):
        """
        Computes the posterior covariance between points.

        :param X1: some input observations
        :param X2: other input observations
        """
        return self.posterior.covariance_between_points(self.kern, self.X, X1, X2)