title: Add Missing Columns to the dataframe
body: <p>I have a list of dataframes</p>
  
  <p>dfA:</p>
  
  <pre><code>item   a     A              
  A      1     2 
  B      1     3         
  C      0     4         
  </code></pre>
  
  <p>dfB:</p>
  
  <pre><code>item   a     B
  E      1     2
  F      0     6
  </code></pre>
  
  <p>dfC:</p>
  
  <pre><code>item   a     C
  G      1     3
  H      0     4
  </code></pre>
  
  <p>I want to add the missing columns to each dataframe</p>
  
  <p>This is what I want :
  dfA:</p>
  
  <pre><code>item   a     A    B    C           
  A      1     2    0    0
  B      1     3    0    0 
  C      0     4    0    0 
  </code></pre>
  
  <p>dfB:</p>
  
  <pre><code>item   a     A   B    C
  E      1     0   2    0
  F      0     0   6    0
  </code></pre>
  
  <p>dfC:</p>
  
  <pre><code>item   a     A   B   C
  G      1     0   0   3
  H      0     0   0   4
  </code></pre>