title: Flatten 3-d numpy array
body: <p>How to flatten this:</p>
  
  <pre><code>b = np.array([
      [[1,2,3], [4,5,6], [7,8,9]],
      [[1,1,1],[2,2,2],[3,3,3]]
  ])
  </code></pre>
  
  <p>into:</p>
  
  <pre><code>c = np.array([
      [1,2,3,4,5,6,7,8,9],
      [1,1,1,2,2,2,3,3,3]
  ])
  </code></pre>
  
  <p>Niether of these work:</p>
  
  <pre><code>c = np.apply_along_axis(np.ndarray.flatten, 0, b)
  c = np.apply_along_axis(np.ndarray.flatten, 0, b)
  </code></pre>
  
  <p>Just returns the same array.</p>
  
  <p>It would be great to flatten this in place.</p>