docs: def acor(self, k = 5):
          """
      Autocorrelation time of the chain
          return the autocorrelation time for each parameters
      Inputs :
          k : 
              parameter in self-consistent window
      Outputs :
          t :
              autocorrelation time of the chain
          """
          try:
              import acor
          except ImportError:
              print("Can't import acor, please download it.")
              return 0
          n = np.shape(self._chain)[1]
          t = np.zeros(n)
          for i in xrange(n):
              t[i] = acor.acor(self._chain[:,i],k)[0]
          return t