title: How to ignore values when using numpy.sum and numpy.mean in matrices
body: <p>Is there a way to avoid using specific values when applying sum and mean in numpy?</p>
  
  <p>I'd like to avoid, for instance, the -999 value when calculating the result.</p>
  
  <pre><code>In [14]: c = np.matrix([[4., 2.],[4., 1.]])
  
  In [15]: d = np.matrix([[3., 2.],[4., -999.]])
  
  In [16]: np.sum([c, d], axis=0)
  Out[16]:
  array([[   7.,    4.],
         [   8., -998.]])
  
  In [17]: np.mean([c, d], axis=0)
  Out[17]:
  array([[   3.5,    2. ],
         [   4. , -499. ]])
  </code></pre>