title: Add together two numpy masked arrays
body: <p>Is there a convenient way to add another array with actual values to masked positions in another array?</p>
  
  <pre><code>import numpy as np
  arr1 = np.ma.array([0,1,0], mask=[True, False, True])
  arr2 = np.ma.array([2,3,0], mask=[False, False, True])
  arr1+arr2
  
  Out[4]:
  masked_array(data = [-- 4 --],
           mask = [ True False  True],
     fill_value = 999999)
  </code></pre>
  
  <p>Note: in arr2 the value 2 is not masked -> should be in the resulting array
  The result should be [2, 4, --]. I'd think there must be an easy solution for this?</p>