title: Concatenate two numpy arrays in the 4th dimension
body: <p>I have two numpy arrays with three dimensions (3 x 4 x 5) and I want to concatenate them so the result has four dimensions (3 x 4 x 5 x 2). In Matlab, this can be done with <code>cat(4, a, b)</code>, but not in Numpy.</p>
  
  <p>For example:</p>
  
  <pre><code>a = ones((3,4,5))
  b = ones((3,4,5))
  c = concatenate((a,b), axis=3) # error!
  </code></pre>
  
  <p>To clarify, I wish <code>c[:,:,:,0]</code> and <code>c[:,:,:,1]</code> to correspond to the original two arrays.</p>