title: pandas pivot and join in two dataframes
body: <p>I have two dataFrames :</p>
  
  <pre><code>df1
     mag   cat
  0  101   A1
  1  256   A2  
  2  760   A2
  3  888   A3  
  ...
  
  df2
     A1    A2    A3    ...
  0  E50R  AZ33  REZ3 
  1  T605  YYU6  YHG5
  2  IR50  P0O9  BF53
  3  NaN   YY9I  NaN
  </code></pre>
  
  <p>And I would like to create a final DataFrame which looks like :</p>
  
  <pre><code>df
     101   256   760   888  ...
  0  E50R  AZ33  AZ33  REZ3
  1  T605  YYU6  YYU6  YHG5
  2  IR50  P0O9  P0O9  BF53
  3  NaN   YY9I  YY9I  NaN
  </code></pre>
  
  <p>I tried something with pivot, but it doesn't seem to do the job
  Could you help me ?</p>