title: Pandas cumulative sum of all previous dates by group
body: <p>I have a Pandas dataframe like this:</p>
  
  <pre><code>df = pd.DataFrame({
      'Date': ['2018-04-01', '2018-05-01', '2018-06-01', '2018-07-01', '2018-08-01'],
      'Product': ['a', 'a', 'a', 'b', 'b'],
      'Volumes': [10,30,40,50,60]})
  
  Date       Product   Volumes
  2018-04-01  a        10
  2018-05-01  a        30
  2018-06-01  a        40
  2018-07-01  b        50
  2018-08-01  b        60
  
  </code></pre>
  
  <p>I want to create a column which sums all the volumes of all preceding months for the same product only. </p>
  
  <p>The desired output would be: </p>
  
  <pre><code>
  df = pd.DataFrame({
      'Date': ['2018-04-01', '2018-05-01', '2018-06-01', '2018-07-01', '2018-08-01'],
      'Product': ['a', 'a', 'a', 'b', 'b'],
      'Volumes': [10,30,40,50,60],
      'Result': [10,40,80,50,110] })
  
  
  Date        Product   Volumes   Result
  2018-04-01   a         10       10
  2018-05-01   a         30       40
  2018-06-01   a         40       80
  2018-07-01   b         50       50
  2018-08-01   b         60       110
  </code></pre>
  
  <p>Does anyone have any advice on how to do this? </p>
  
  <p>Thanks!</p>