title: Crop part of np.array
body: <p>Ihave a numpy array A like</p>
  
  <pre><code>A.shape
  (512,270,1,20)
  </code></pre>
  
  <p>I dont want to use all the 20 layers in dimension 4. The new array should be like</p>
  
  <pre><code>Anew.shape
  (512,270,1,2)
  </code></pre>
  
  <p>So I want to crop out 2 "slices" of the array A</p>