Add predictions and confidence interval to self.df
        self.df will contain the following columns:
        - 'predicted': the model output
        - 'interval_u', 'interval_l': upper and lower confidence bounds.

        Parameters
        ----------
        None, but the result depends on the following attributes of self:
        confint : float (default=0.95)
            Confidence level for two-sided hypothesis
        allow_negative_predictions : bool (default=True)
            If False, correct negative predictions to zero (typically for energy consumption predictions)

        Returns
        -------
        Nothing, adds columns to self.df

def add_prediction(self):
        """
        Add predictions and confidence interval to self.df
        self.df will contain the following columns:
        - 'predicted': the model output
        - 'interval_u', 'interval_l': upper and lower confidence bounds.

        Parameters
        ----------
        None, but the result depends on the following attributes of self:
        confint : float (default=0.95)
            Confidence level for two-sided hypothesis
        allow_negative_predictions : bool (default=True)
            If False, correct negative predictions to zero (typically for energy consumption predictions)

        Returns
        -------
        Nothing, adds columns to self.df
        """
        self.df = self._predict(fit=self.fit, df=self.df)