private method exponential 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 ExpVario

def _h_function(self,h):
        """ private method exponential 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 ExpVario

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