Creates a h-step ahead mean prediction

        Parameters
        ----------
        theta : np.array
            The past local level

        theta_t : np.array
            The past local linear trend

        Y : np.array
            The past data

        scores : np.array
            The past scores

        h : int
            How many steps ahead for the prediction

        t_params : np.array
            A vector of (transformed) latent variables

        Returns
        ----------
        Y_exp : np.array
            Vector of past values and predictions

def _mean_prediction(self, theta, theta_t, Y, scores, h, t_params):
        """ Creates a h-step ahead mean prediction

        Parameters
        ----------
        theta : np.array
            The past local level

        theta_t : np.array
            The past local linear trend

        Y : np.array
            The past data

        scores : np.array
            The past scores

        h : int
            How many steps ahead for the prediction

        t_params : np.array
            A vector of (transformed) latent variables

        Returns
        ----------
        Y_exp : np.array
            Vector of past values and predictions 
        """     

        Y_exp = Y.copy()
        theta_exp = theta.copy()
        theta_t_exp = theta_t.copy()
        scores_exp = scores.copy()

        #(TODO: vectorize the inner construction here)   
        for t in range(0,h):
            new_value1 = theta_t_exp[-1] + theta_exp[-1] + t_params[0]*scores_exp[-1]
            new_value2 = theta_t_exp[-1] + t_params[1]*scores_exp[-1]
            if self.model_name2 == "Exponential":
                Y_exp = np.append(Y_exp, [1.0/self.link(new_value1)])
            else:
                Y_exp = np.append(Y_exp, [self.link(new_value1)])
            theta_exp = np.append(theta_exp, [new_value1]) # For indexing consistency
            theta_t_exp = np.append(theta_t_exp, [new_value2])
            scores_exp = np.append(scores_exp, [0]) # expectation of score is zero
        return Y_exp