title: convenient way to reindex one level of a multiindex
body: <p>consider the <code>pd.Series</code> <code>s</code> and <code>pd.MultiIndex</code> <code>idx</code></p>
  
  <pre><code>idx = pd.MultiIndex.from_product([list('AB'), [1, 3], list('XY')],
                                   names=['one', 'two', 'three'])
  s = pd.Series(np.arange(8), idx)
  s
  
  one  two  three
  A    1    X        0
            Y        1
       3    X        2
            Y        3
  B    1    X        4
            Y        5
       3    X        6
            Y        7
  dtype: int32
  </code></pre>
  
  <p>I want to <code>reindex</code> on <code>level='two'</code> with <code>np.arange(4)</code><br>
  I can do it with:</p>
  
  <pre><code>s.unstack([0, 2]).reindex(np.arange(4), fill_value=0).stack().unstack([0, 1])
  
  one  two  three
  A    0    X        0
            Y        0
       1    X        0
            Y        1
       2    X        0
            Y        0
       3    X        2
            Y        3
  B    0    X        0
            Y        0
       1    X        4
            Y        5
       2    X        0
            Y        0
       3    X        6
            Y        7
  dtype: int32
  </code></pre>
  
  <p>But I'm looking for something more direct if it exists.  Any ideas?</p>