code: def X(self):
               
          if self.ar == 1:
              return np.array([self.data_full[(self.max_lag-1):-1]])
          else:
              for i in range(0,self.ar):
                  datapoint = self.data_full[(self.max_lag-i-1):-i-1]         
                  if i == 0:
                      X = datapoint
                  else:
                      X = np.vstack((X,datapoint))
          return X
docs: Creates design matrix of variables to use in GP regression
          
          Returns
          ----------
          The design matrix