title: Rename pandas column values from unstacked pivot table
body: <p>I have pandas pivot table <code>merge2</code> that looks like:</p>
  
  <pre><code>   Site   TripDate       Volume    Early_Vol  Percent_Vol
  0  024l 2004-12-02  1117.134948  1117.134948     0.000000
  1  024l 2005-05-07   390.980708  1117.134948    -0.650015
  2  024l 2006-10-07   321.110175  1117.134948    -0.712559
  3  024l 2007-10-13   527.631767  1117.134948    -0.527692
  4  024l 2008-02-02   597.165065  1117.134948    -0.465449
  </code></pre>
  
  <p>I then subset <code>merge2</code> into <code>sub1</code></p>
  
  <pre><code>sub1 = merge2[['Site','TripDate','Percent_Vol']]
  sub1= sub1.set_index(['Site','TripDate'])
  </code></pre>
  
  <p><code>sub1</code> looks like:</p>
  
  <pre><code>             Percent_Vol
  Site TripDate               
  024l 2004-12-02     0.000000
       2005-05-07    -0.650015
       2006-10-07    -0.712559
       2007-10-13    -0.527692
       2008-02-02    -0.465449
  </code></pre>
  
  <p>I then unstack <code>sub1</code> into <code>t</code> to look like:</p>
  
  <pre><code>           Percent_Vol                                                
  Site              024l      029l      033l 035l_r    035l_s    041r_r   
  TripDate                                                                
  2004-06-01         NaN       NaN       NaN    NaN       NaN       NaN   
  2004-11-13         NaN       NaN       NaN    NaN       NaN       NaN   
  2004-12-02    0.000000  0.000000  0.000000    0.0  0.000000  0.000000   
  2005-05-07   -0.650015 -0.290539 -0.300276   -1.0 -0.075511 -0.298801   
  2006-10-07   -0.712559 -0.769020 -0.393304   -1.0 -0.520052 -0.284686  
  </code></pre>
  
  <p>It seems like I have two levels for my columns names in <code>t</code> (i.e. <code>Percent_Vol</code> and <code>Site</code>) and I only what <code>Site</code>.  Is there a way to change the column names to just <code>Site</code>?</p>