Creates the Autoregressive matrix for the model

        Returns
        ----------
        X : np.ndarray
            Autoregressive Matrix

def _ar_matrix(self):
        """ Creates the Autoregressive matrix for the model

        Returns
        ----------
        X : np.ndarray
            Autoregressive Matrix
        """

        X = np.ones(self.data_length-self.max_lag)

        if self.ar != 0:
            for i in range(0, self.ar):
                X = np.vstack((X,self.data[(self.max_lag-i-1):-i-1]))

        return X