title: accumulate numpy array for just one column
body: <p>I have a NumPy array, i want to accumulate the values of one column, say the 2<sup>nd</sup> column.</p>
  
  <pre><code>a = np.array([[1,2],[2,4]])
  # some kind of accumulate function that accumulates just one column:
  np.add.accumulate(a, 2)
  </code></pre>
  
  <p><code>a</code> should now be <code>[[1,2],[2,6]]</code></p>
  
  <p>Is there a way to do this in NumPy?</p>