get the covariance between base point x0,y0 and
        other points xother,yother implied by Vario2d

        Parameters
        ----------
        x0 : (float)
            x-coordinate of base point
        y0 : (float)
            y-coordinate of base point
        xother : (float or numpy.ndarray)
            x-coordinates of other points
        yother : (float or numpy.ndarray)
            y-coordinates of other points

        Returns
        -------
        cov : numpy.ndarray
            covariance between base point and other points implied by
            Vario2d.

        Note
        ----
        len(cov) = len(xother) = len(yother)

def covariance_points(self,x0,y0,xother,yother):
        """ get the covariance between base point x0,y0 and
        other points xother,yother implied by Vario2d

        Parameters
        ----------
        x0 : (float)
            x-coordinate of base point
        y0 : (float)
            y-coordinate of base point
        xother : (float or numpy.ndarray)
            x-coordinates of other points
        yother : (float or numpy.ndarray)
            y-coordinates of other points

        Returns
        -------
        cov : numpy.ndarray
            covariance between base point and other points implied by
            Vario2d.

        Note
        ----
        len(cov) = len(xother) = len(yother)

        """
        dx = x0 - xother
        dy = y0 - yother
        dxx,dyy = self._apply_rotation(dx,dy)
        h = np.sqrt(dxx*dxx + dyy*dyy)
        return self._h_function(h)