title: Combine Rows of Multiindex DataFrame into Comma Separated Lists
body: <p>Given a multi-index DataFrame, I would like to combine repeated index pairs and list their values as comma-separated lists. For example, the input:</p>
  
  <pre><code>df = pd.DataFrame({'Last Name' : ['Deere','Deere','Foo'   ,'Foo'  ,'Man'   ],
                     'First Name': ['John' ,'Jane' ,'Kung'  ,'Kung' ,'Karate'],
                     'Value1':     [ 1     , 2     , 3      , 4     , 5      ],
                     'Value2':     ['Green','Blue' ,'Yellow','Black','Purple']})
  
  df.set_index(['Last Name','First Name'],inplace=True)
  </code></pre>
  
  <p>Provides:</p>
  
  <pre><code>                          Value1    Value2
  Last Name   First Name      
  Deere       John          1         Green
              Jane          2         Blue
  Foo         Kung          3         Yellow
              Kung          4         Black
  Man         Karate        5         Purple
  </code></pre>
  
  <p>I would like to convert this into the following DataFrame:</p>
  
  <pre><code>                          Value1    Value2
  Last Name   First Name      
  Deere       John          1         Green
              Jane          2         Blue
  Foo         Kung          3,4       Yellow,Black
  Man         Karate        5         Purple
  </code></pre>