title: Expand/Contract a numpy matrix with None
body: <p>How can I expand or contract a numpy matrix and populate it with None?</p>
  <p>My expected behaviour is:</p>
  <pre class="lang-py prettyprint-override"><code>Y = np.arange(16).reshape(4,4)
  
  # Transform it to (7, 3)
  array([[0, 1, 2],
         [4, 5, 6],
         [8, 9, 10],
         [12, 13, 14],
         [None, None, None],
         [None, None, None],
         [None, None, None]], dtype=object)
  
  # Transform it to (2, 3)
  array([[0, 1, 2],
         [4, 5, 6]], dtype=object)
  
  </code></pre>
  <p>Basically, retain the submatrix and if the dimensions are larger populate them with None.</p>
  <p>Is there a vectorized way to do this?</p>