title: Fast in-place replacement of some values in a numpy array
body: <p>There has got to be a faster way to do in place replacement of values, right?  I've got a 2D array representing a grid of elevations/bathymetry.  I want to replace anything over 0 with NAN and this way is super slow:</p>
  
  <pre><code>for x in range(elevation.shape[0]):
      for y in range(elevation.shape[1]):
          if elevation[x,y] &gt; 0:
              elevation[x,y] = numpy.NAN
  </code></pre>
  
  <p>It seems like that has so be a much better way!</p>