title: replace masked with nan in numpy masked_array
body: <pre><code>&gt;&gt; masks = [[1,1],[0,0]]    
  &gt;&gt; [np.ma.masked_array(data=np.array([1.0,2.0]), mask=m, fill_value=np.nan).mean() for m in masks]
     [masked, 1.5]
  </code></pre>
  
  <p>I'd like to replace the <code>masked</code> result with <code>nan</code>. Is there a way to do that directly with numpy's <code>masked_array</code>?</p>