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

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