title: Python Running cumulative sum with a given window
body: <p>What I want to do is generate a numpy array that is the cumulative sum of another numpy array given a certain window. </p>
  
  <p>For example, given an array <code>[1,2,3,4,5,6,7,8,9,10,11,12]</code> let's say I want a cumulative sum with a window of 3. What I want as out put would be <code>[1,3,6,9,12,15,18,21,24,27,30,33]</code>. I have a relatively large numpy array and would like to do a cumulative sum with a window of 400.</p>