title: "unstack" a pandas column containing lists into multiple rows
body: <p>Say I have the following Pandas Dataframe:</p>
  
  <pre><code>df = pd.DataFrame({"a" : [1,2,3], "b" : [[1,2],[2,3,4],[5]]})
     a          b
  0  1     [1, 2]
  1  2  [2, 3, 4]
  2  3        [5]
  </code></pre>
  
  <p>How would I "unstack" the lists in the "b" column in order to transform it into the dataframe:</p>
  
  <pre><code>   a  b
  0  1  1
  1  1  2
  2  2  2
  3  2  3
  4  2  4
  5  3  5
  </code></pre>