GAS Normal Update term using gradient only - native Python function

        Parameters
        ----------
        y : float
            datapoint for the time series

        mean : float
            location parameter for the Normal distribution

        scale : float
            scale parameter for the Normal distribution

        shape : float
            tail thickness parameter for the Normal distribution

        skewness : float
            skewness parameter for the Normal distribution

        Returns
        ----------
        - Score of the Normal family

def first_order_score(y, mean, scale, shape, skewness):
        """ GAS Normal Update term using gradient only - native Python function

        Parameters
        ----------
        y : float
            datapoint for the time series

        mean : float
            location parameter for the Normal distribution

        scale : float
            scale parameter for the Normal distribution

        shape : float
            tail thickness parameter for the Normal distribution

        skewness : float
            skewness parameter for the Normal distribution

        Returns
        ----------
        - Score of the Normal family
        """
        return (y-mean)/np.power(scale,2)