title: Numpy integer nan
body: <p>Is there a way to store NaN in a Numpy array of integers?
  I get: </p>
  
  <pre><code>a=np.array([1],dtype=long)
  a[0]=np.nan
  
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  ValueError: cannot convert float NaN to integer
  </code></pre>