title: NumPy k-th diagonal indices
body: <p>I'd like to do arithmetics with k-th diagonal of a numpy.array. I need those indices.
  For example, something like:</p>
  
  <pre><code>&gt;&gt;&gt; a = numpy.eye(2)
  &gt;&gt;&gt; a[numpy.diag_indices(a, k=-1)] = 5
  &gt;&gt;&gt; a
  array([[ 1.,  0.],
         [ 5.,  1.]])
  </code></pre>
  
  <p>Unfortunately, diag_indices only returns the indices comprising the main diagonal, so at the moment I am doing:</p>
  
  <pre><code>a += numpy.diag([5], -1)
  </code></pre>
  
  <p>But that doesn't seem as nice or robust. :-)</p>
  
  <p>Is there a way in numpy to get indices for other than the main diagonal?</p>