title: Masked `np.nan` in the `np.ma.array` problem in jupyter
body: <p>Let's run in the Anaconda Jupyter the Python3 NumPy code:</p>
  
  <pre><code>y = np.ma.array(np.matrix([[np.nan, 2.0]]), mask=[0, 1])
  m = (y &lt; 0.01)
  </code></pre>
  
  <p>and we have the warning: <code>/.../anaconda3/lib/python3.7/site-packages/ipykernel_launcher.py:2: RuntimeWarning: invalid value encountered in less</code>.</p>
  
  <p>Substituting <code>np.nan</code> with <code>1.0</code> etc. --- no warning.</p>
  
  <p>Why the <code>np.nan</code> can not be masked and then compared?</p>