title: Get original indices of a sorted Numpy array
body: <p>I have an array of distances <code>a = np.array([20.5 ,5.3 ,60.7 ,3.0 ], 'double')</code> and I need the indices of the sorted array (for example <code>[3, 1, 0, 2]</code>, for <code>a.sort()</code>). Is there a function in Numpy to do that?</p>