code: def _ar_matrix(self):
          
  
          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
docs: Creates the Autoregressive matrix for the model
  
          Returns
          ----------
          X : np.ndarray
              Autoregressive Matrix