get the sill of the GeoStruct

        Return
        ------
        sill : float
            the sill of the (nested) GeoStruct, including nugget and contribution
            from each variogram

def sill(self):
        """ get the sill of the GeoStruct

        Return
        ------
        sill : float
            the sill of the (nested) GeoStruct, including nugget and contribution
            from each variogram
        """
        sill = self.nugget
        for v in self.variograms:
            sill += v.contribution
        return sill