title: Unexpected behaviour when indexing a 2D np.array with two boolean arrays
body: <pre><code>two_d = np.array([[ 0,  1,  2,  3,  4],
                    [ 5,  6,  7,  8,  9],
                    [10, 11, 12, 13, 14],
                    [15, 16, 17, 18, 19],
                    [20, 21, 22, 23, 24]])
  
  first = np.array((True, True, False, False, False))
  second = np.array((False, False, False, True, True))
  </code></pre>
  
  <p>Now, when I enter:</p>
  
  <pre><code>two_d[first, second]
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>array([3,9])
  </code></pre>
  
  <p>which doesn't make a whole lot of sense to me. Can anybody explain that simply?</p>