get the covarince between two points implied by Vario2d

        Parameters
        ----------
        pt0 : (iterable of len 2)
            first point x and y
        pt1 : (iterable of len 2)
            second point x and y

        Returns
        -------
        cov : float
            covariance between pt0 and pt1

def covariance(self,pt0,pt1):
        """ get the covarince between two points implied by Vario2d

        Parameters
        ----------
        pt0 : (iterable of len 2)
            first point x and y
        pt1 : (iterable of len 2)
            second point x and y

        Returns
        -------
        cov : float
            covariance between pt0 and pt1

        """

        x = np.array([pt0[0],pt1[0]])
        y = np.array([pt0[1],pt1[1]])
        names = ["n1","n2"]
        return self.covariance_matrix(x,y,names=names).x[0,1]