title: Reindex sublevel of pandas dataframe multiindex
body: <p>I have a time series dataframe and I would like to reindex it by Trials and Measurements.</p>
  
  <p>Simplified, I have this:</p>
  
  <pre><code>                value
  Trial         
      1     0        13
            1         3
            2         4
      2     3       NaN
            4        12
      3     5        34   
  </code></pre>
  
  <p>Which I want to turn into this:</p>
  
  <pre><code>                  value
  Trial    
      1      0        13
             1         3
             2         4
      2      0       NaN
             1        12
      3      0        34
  </code></pre>
  
  <p>How can I best do this?   </p>