title: Merging rows in a dataframe depending on another column
body: <p>I have extracted a pdf into a dataframe and would like to merge the rows if column B is the same speaker :</p>
  
  <p>From : </p>
  
  <pre><code>  Index     Column B     Column C 
     1       'I am going'    Speaker A 
     2       'to the zoo'    Speaker A
     3       'I am going'    Speaker B 
     4       'home      '    Speaker B
     5       'I am going'    Speaker A 
     6       'to the park'   Speaker A
  </code></pre>
  
  <p>To : </p>
  
  <pre><code>  Index     Column B                    Column C 
     1       'I am going to the zoo '    Speaker A 
     2       'I am going home'           Speaker B
     3       'I am going to the park'    Speaker A 
  </code></pre>
  
  <p>I tried using groupby but the order is important in the context of a pdf which is a speech. </p>