title: How to sort a numpy array on a specific row, with other rows to change accordingly?
body: <p>I have a numpy array <code>a</code> like this</p>
  <pre><code>In [318]: a
  Out[318]:
  array([[0. , 1. , 2. , 3. ],
         [0.5, 0.3, 0.2, 0.25]])
  </code></pre>
  <p>I need to sort along the second row (the one with [0.5,0.3,0.2,0.25]), while having the first row changed accordingly. In this case, the expected result is</p>
  <pre><code>2    3     1.   0
  0.2, 0.25, 0.3, 0.5
  </code></pre>
  <p>How can I do this? Thank you. I tried np.sort with axis=-1 and 0; they are not what I need.</p>
  <p>Important note: The performance is the key in my problem solving. My arrays, from an image processing application, are generally of N columns with an N close to 4 million.</p>