title: Pandas Sort Multiindex by Group Sum
body: <p>Given the following data frame:</p>
  
  <pre><code>import pandas as pd
  df=pd.DataFrame({'County':['A','B','C','D','A','B','C','D','A','B','C','D','A','B','C','D','A','B'],
                  'Hospital':['a','b','c','d','e','a','b','c','e','a','b','c','d','e','a','b','c','e'],
                  'Enrollment':[44,55,42,57,95,54,27,55,81,54,65,23,89,76,34,12,1,67],
                  'Year':['2012','2012','2012','2012','2012','2012','2012','2012','2012','2013',
                          '2013','2013','2013','2013','2013','2013','2013','2013']})
  d2=pd.pivot_table(df,index=['County','Hospital'],columns=['Year'])#.sort_columns
  
  d2
          Enrollment
          Year   2012     2013
  County  Hospital        
  A       a      44.0     NaN
          c      NaN      1.0
          d      NaN      89.0
          e      88.0     NaN
  B       a      54.0     54.0
          b      55.0     NaN
          e      NaN      71.5
  C       a      NaN      34.0
          b      27.0     65.0
          c      42.0     NaN
  D       b      NaN      12.0
          c      55.0     23.0
          d      57.0     NaN
  </code></pre>
  
  <p>I need to sort the data frame such that County is sorted descendingly by the sum of Enrollment for the most recent year (I want to avoid using '2013' directly) like this:</p>
  
  <pre><code>        Enrollment  
      Year          2012  2013
  County  Hospital        
  B       a         54    54
          b         55    NaN
          e         NaN   71.5
  C       a         NaN   34
          b         27    65
          c         42    NaN
  A       a         44    NaN
          c         NaN   1
          d         NaN   89
          e         88    NaN
  D       b         NaN   12
          c         55    23
          d         57    NaN
  </code></pre>
  
  <p>Then, I'd like each hospital sorted within each county, descendingly, but 2013 enrollments like this:</p>
  
  <pre><code>        Enrollment  
          Year    2012    2013
  County  Hospital        
  B       e       NaN 71.5
          a       54  54
          b       55  NaN
  C       b       27  65
          a       NaN 34
          c       42  NaN
  A       d       NaN 89
          c       NaN 1
          a       44  NaN
          e       88  NaN
  D       c       55  23
          b       NaN 12
          d       57  NaN
  </code></pre>
  
  <p>So far, I've tried using groupby to get the sums and merge the back but have not had any luck:</p>
  
  <pre><code>d2.groupby('County').sum()
  </code></pre>
  
  <p>Thanks in advance!</p>