title: numpy unique without sort
body: <p>How can I use numpy unique without sorting the result but just in the order they appear in the sequence? Something like this?</p>
  
  <p><code>a = [4,2,1,3,1,2,3,4]</code></p>
  
  <p><code>np.unique(a) = [4,2,1,3]</code></p>
  
  <p>rather than</p>
  
  <p><code>np.unique(a) = [1,2,3,4]</code></p>
  
  <p>Use naive solution should be fine to write a simple function. But as I need to do this multiple times, are there any fast and neat way to do this?</p>