title: merge and sum two dataframes where columns match python pandas
body: <p>I might be approaching this is the wrong way. If I have two dataframes a and b:</p>
  
  <p>Dataframe A:</p>
  
  <pre><code>a b c
  1 2 4
  1 6 5
  1 8 7
  </code></pre>
  
  <p>and Dataframe B:</p>
  
  <pre><code>a b c d
  1 2 4 9
  1 6 5 7
  1 8 7 10
  </code></pre>
  
  <p>And I want to join them, but cruically sum the columns where the column name matchs, and keep the column names that don't match. So the final joined table would look like:</p>
  
  <pre><code>a   b   c   d
  2   4   8   9
  2   12  10  7
  2   16  14  10
  </code></pre>
  
  <p>Note: The indexs would always match (i.e the same number of records in the same order)</p>