title: Numpy sum over planes of 3d array, return a scalar
body: <p>I'm making the transition from MATLAB to Numpy and feeling some growing pains.</p>
  
  <p>I have a 3D array, lets say it's 3x3x3 and I want the scalar sum of each plane.
  In matlab, I would use:</p>
  
  <pre><code>sum_vec = sum(3dArray,3);
  </code></pre>
  
  <p>TIA
  wbg</p>
  
  <p>EDIT: I was wrong about my matlab code. Matlab only vectorizes in one dim, so a loop wold be required. So numpy turns out to be more elegant...cool.</p>
  
  <pre><code>MATLAB
  for i = 1:3
      sum_vec(i) = sum(sum(3dArray(:,:,i));
  end
  </code></pre>