get the covariance between two points implied by the GeoStruct.
        This is used during the ordinary kriging process to get the RHS

        Parameters
        ----------
        pt0 : (iterable length 2 of floats)
        pt1 : (iterable length 2 of floats)

        Returns
        -------
        covariance : float
            the covariance between pt0 and pt1 implied by the GeoStruct

def covariance(self,pt0,pt1):
        """get the covariance between two points implied by the GeoStruct.
        This is used during the ordinary kriging process to get the RHS

        Parameters
        ----------
        pt0 : (iterable length 2 of floats)
        pt1 : (iterable length 2 of floats)

        Returns
        -------
        covariance : float
            the covariance between pt0 and pt1 implied by the GeoStruct

        """
        #raise Exception()
        cov = self.nugget
        for vario in self.variograms:
            cov += vario.covariance(pt0,pt1)
        return cov