title: How to groupby based on two columns in pandas?
body: <p>A similar question might have been asked before, but I couldn't find the exact one fitting to my problem.
  I want to group by a dataframe based on two columns. 
  For exmaple to make this </p>
  
  <pre><code>id product quantity
  1  A       2
  1  A       3
  1  B       2
  2  A       1
  2  B       1
  3  B       2
  3  B       1
  </code></pre>
  
  <p>Into this:</p>
  
  <pre><code>id product quantity
  1  A       5
  1  B       2
  2  A       1
  2  B       1
  3  B       3
  </code></pre>
  
  <p>Meaning that summation on "quantity" column for same "id" and same "product".</p>