title: Flatten numpy array
body: <p>Is there a simple way in NumPy to flatten type object array?</p>
  
  <p>I know .flatten() method flattens non-object type arrays constructed from same size arrays:</p>
  
  <pre><code>I1 a = np.array([[1],[2],[3]])
  
  I2 a.flatten()
  O2 array([1, 2, 3])
  </code></pre>
  
  <p>however, I can't get dtype=object array flattened:</p>
  
  <pre><code>I4 b
  O4 array([[1], [2, 3], [3]], dtype=object)
  
  I5 b.flatten()
  O5 array([[1], [2, 3], [3]], dtype=object)
  </code></pre>
  
  <p>Thanks.</p>