Compute the mean and variance of held-out data at the points Xnew

def predict_y(self, Xnew):
        """
        Compute the mean and variance of held-out data at the points Xnew
        """
        pred_f_mean, pred_f_var = self._build_predict(Xnew)
        return self.likelihood.predict_mean_and_var(pred_f_mean, pred_f_var)