title: using python.itertools to sum slots across multiple lists
body: <p>I have this:</p>
  
  <pre><code>[
      [ [1,1,1,0,0,0,0], [0,1,2,3,4,5,6], [1,2,3,4,5,6,7], [2,3,4,5,6,7,8], [3,4,5,6,7,8,9], [4,5,6,7,8,9,0] ]
  
      , [ [2,2,2,0,0,0,0], [0,1,2,3,4,5,6], [1,2,3,4,5,6,7], [2,3,4,5,6,7,8], [3,4,5,6,7,8,9], [4,5,6,7,8,9,2] ]
  
      , [ [3,3,3,0,0,0,0], [0,1,2,3,4,5,6], [1,2,3,4,5,6,7], [2,3,4,5,6,7,8], [3,4,5,6,7,8,9], [4,5,6,7,8,9,0] ]
  ]
  </code></pre>
  
  <p>and I'm trying to get this:</p>
  
  <pre><code>[
  [[6, 6, 6, 0, 0, 0, 0], [0, 3, 6, 9, 12, 15, 18], [3, 6, 9, 12, 15, 18, 21], [6, 9, 12, 15, 18, 21, 24], [9, 12, 15, 18, 21, 24, 27], [12, 15, 18, 21, 24, 27, 2]]
  ]
  </code></pre>
  
  <p>it's just a sum of all of the relative positions inside the deepest lists.
  Right now, I've got 3 nested for-loops and I'm hoping one of you itertools experts can show me a faster &amp; more elegant way to do this </p>