title: Compute mean of a 3D numpy array
body: <p>I have a <code>3D numpy array</code> (called data) of shape <code>(600L, 24L, 351L)</code>.
  The first dimension <code>(600)</code> represents the <code>number of epochs</code>.
  The second dimension <code>(24)</code> represents the <code>number of channels</code>.</p>
  
  <p>The last dimension represents voltage values over a period of time of <code>351 ms</code> (one voltage value per millisecond). </p>
  
  <p>For each of the 24 channels and 351 time steps, I would like to average voltage values over the 600 epochs. </p>
  
  <p>I would thus get a numpy array of shape (24, 351).
  I guess I need to write something like:</p>
  
  <p><code>mean_data = np.mean(data, axis = (some sort of tuple))</code>,</p>
  
  <p>but I can't figure out the tuple needed for the axis argument. Any help would be appreciated. </p>