private method for the spherical variogram "h" function

        Parameters
        ----------
        h : (float or numpy.ndarray)
            distance(s)

        Returns
        -------
        h_function : float or numpy.ndarray
            the value of the "h" function implied by the SphVario

def _h_function(self,h):
        """ private method for the spherical variogram "h" function

        Parameters
        ----------
        h : (float or numpy.ndarray)
            distance(s)

        Returns
        -------
        h_function : float or numpy.ndarray
            the value of the "h" function implied by the SphVario

        """

        hh = h / self.a
        h = self.contribution * (1.0 - (hh * (1.5 - (0.5 * hh * hh))))
        h[hh > 1.0] = 0.0
        return h