Get the covariance between point x0,y0 and the points
        contained in xother, yother.

        Parameters
        ----------
        x0 : (float)
            x-coordinate
        y0 : (float)
            y-coordinate
        xother : (iterable of floats)
            x-coordinate of other points
        yother : (iterable of floats)
            y-coordinate of other points

        Returns
        -------
        cov : numpy.ndarray
            a 1-D array of covariance between point x0,y0 and the
            points contained in xother, yother.  len(cov) = len(xother) =
            len(yother)

def covariance_points(self,x0,y0,xother,yother):
        """ Get the covariance between point x0,y0 and the points
        contained in xother, yother.

        Parameters
        ----------
        x0 : (float)
            x-coordinate
        y0 : (float)
            y-coordinate
        xother : (iterable of floats)
            x-coordinate of other points
        yother : (iterable of floats)
            y-coordinate of other points

        Returns
        -------
        cov : numpy.ndarray
            a 1-D array of covariance between point x0,y0 and the
            points contained in xother, yother.  len(cov) = len(xother) =
            len(yother)

        """

        cov = np.zeros((len(xother))) + self.nugget
        for v in self.variograms:
            cov += v.covariance_points(x0,y0,xother,yother)
        return cov