private method for the gaussian 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 GauVario

def _h_function(self,h):
        """ private method for the gaussian 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 GauVario

        """

        hh = -1.0 * (h * h) / (self.a * self.a)
        return self.contribution * np.exp(hh)