title: add rows in pandas dataframe based on date and value
body: <p>I have a pandas dataframe like the following:</p>
  
  <pre><code>id, date, add_days
  1, 2017-01-01, 3
  2, 2017-03-05, 5
  3, 2017-02-27, 3
  .
  .
  .
  </code></pre>
  
  <p>I want to repeat the ids and increase date by given add_days:</p>
  
  <pre><code>id, date, add_days
  1, 2017-01-01, 3
  1, 2017-01-02, 3
  1, 2017-01-03, 3
  2, 2017-03-05, 5
  2, 2017-03-06, 5
  2, 2017-03-07, 5
  2, 2017-03-08, 5
  2, 2017-03-09, 5
  3, 2017-02-27, 3
  3, 2017-02-28, 3
  3, 2017-03-01, 3
  .
  .
  .
  </code></pre>
  
  <p>Is there a panda-oic way of doing this?
  I'm looking for an efficient solution since the initial dataframe can have millions of rows.</p>