title: numpy: unpack array along an axis
body: <p>Say that I have a RGB image:</p>
  
  <pre><code>from skimage import data
  img = data.astronaut()
  print(img.shape)  # (512, 512, 3)
  </code></pre>
  
  <p>Is there a succinct numpy command to unpack it along the color channels:</p>
  
  <pre><code>R, G, B = np.unpack(img, 2)  # ?
  </code></pre>
  
  <hr>
  
  <p>What I am doing is using comprehension:</p>
  
  <pre><code>R, G, B = (img[:, :, i] for i in range(3))
  </code></pre>
  
  <p>But is there no simpler command?</p>