title: get indicies of non-zero elements of 2D array
body: <p>From <a href="https://stackoverflow.com/questions/13598879/getting-indices-of-both-zero-and-nonzero-elements-in-array">Getting indices of both zero and nonzero elements in array</a>, I can get indicies of non-zero elements in a 1 D array in numpy like this:</p>
  
  <p><code>indices_nonzero = numpy.arange(len(array))[~bindices_zero]</code></p>
  
  <p>Is there a way to extend it to a 2D array?</p>