title: Average columns in 3d array
body: <p>I have a list like</p>
  <pre><code>list = [[[3, 10, 8.53, 0.46], [3, 11, 7.02, 0.53]],
   [[3, 10, 9.03, 0.42], [3, 11, 7.54, 0.55]]]
  </code></pre>
  <p>I want to do the average so that I get a list like:</p>
  <pre><code>[[3, 10, 8.78, 0.44], [3, 11, 7.28, 0.54]]
  </code></pre>
  <p>The first and second columns are the same so I want to keep the integer values. What I'm doing right now is giving me the first two columns as floats. I'd also like to convert the result back to a list.</p>
  <pre><code>&gt;&gt;&gt; a = np.array(list)
  &gt;&gt;&gt; numpy.mean(a, axis=0)
  [[ 3.   10.    8.78  0.44]
   [ 3.   11.    7.28  0.54]]
  </code></pre>