title: Sum rows where value equal in column
body: <p>How can I sum across rows that have equal values in the first column of a numpy array? For example: </p>
  
  <pre><code>In: np.array([[1,2,3],
               [1,4,6], 
               [2,3,5],
               [2,6,2],
               [3,4,8]])
  
  Out: [[1,6,9], [2,9,7], [3,4,8]]
  </code></pre>
  
  <p>Any help would be greatly appreciated.</p>