title: select first column in a 3d ndarray
body: <p>I got a simple question for python.
  Instead of iterating over the first line for a 3D <code>ndarray</code>, I want to select the whole column.
  So lets say:instead of:</p>
  
  <pre><code>print test[0][20][33]
  print test[1][20][33]
  ...
  </code></pre>
  
  <p>I want to put something like:</p>
  
  <pre><code> print test[:][20][33]
  </code></pre>
  
  <p>But this does not work. How do I do it?</p>