title: Reshape dataframe by string variable
body: <p>I have a dataframe as below</p>
  
  <pre><code>df = pd.DataFrame({'a': ['aaa\nbbb\nccc','ddd\nfff\nggg'], 'b':[1,2]})
  df
  </code></pre>
  
  <p>I want to get</p>
  
  <pre><code>     a  b
  0   aaa 1
  1   bbb 1
  2   ccc 1
  3   ddd 2
  4   fff 2
  5   ggg 2
  </code></pre>
  
  <p>I understand how to make it in <code>R</code> with <code>library(reshape)</code> but I cann't fint the analog in <code>python</code></p>