Second derivative of the power transform

        Parameters
        ----------
        p : array-like
            Mean parameters

        Returns
        --------
        g''(p) : array
            Second derivative of the power transform of `p`

        Notes
        -----
        g''(`p`) = `power` * (`power` - 1) * `p`**(`power` - 2)

def deriv2(self, p):
        """
        Second derivative of the power transform

        Parameters
        ----------
        p : array-like
            Mean parameters

        Returns
        --------
        g''(p) : array
            Second derivative of the power transform of `p`

        Notes
        -----
        g''(`p`) = `power` * (`power` - 1) * `p`**(`power` - 2)
        """
        return self.power * (self.power - 1) * np.power(p, self.power - 2)