title: Groupby and append lists and strings
body: <p>I am trying to group-by the values in my "value_1" column. But my last column is made up of lists. When I try to group-by using my "value_1" column, the column made up of lists disappears. </p>
  
  <p>Dataframe: </p>
  
  <pre><code> value_1:        value_2:           value_3:               list: 
   american     california, nyc      walmart, kmart      [supermarket, connivence] 
   canadian         toronto            dunkinDonuts      [coffee]
   american          texas                               [state]
   canadian                             walmart          [supermarket] 
     ...              ...                 ...              ....
  </code></pre>
  
  <p>My expected output is:</p>
  
  <pre><code>value_1:        value_2:              value_3:             list: 
  american   california, nyc, texas   walmart, kmart      [supermarket, connivence, state] 
  canadian         toronto         dunkinDonuts, walmart  [coffee, supermarket]
  </code></pre>
  
  <p>Thanks!</p>