title: pandas dataframe add multiple rows for group of values with apply
body: <p>I have a pandas dataframe contains the following columns:</p>
  
  <pre><code>sate_1      state_2 
  -----       -----
  New York    Washington
  Ohio        Utah
  </code></pre>
  
  <p>what I want to do is to construct an input date for each distinct combination of state_1 and state_2 so the results would be:</p>
  
  <pre><code>sate_1       state_2          date_time
  New York     Washington     2017-11-01 00:00
  New York     Washington     2017-11-01 03:00
  New York     Washington     2017-11-01 06:00
  Ohio         Utah           2017-11-01 00:00
  Ohio         Utah           2017-11-01 03:00
  Ohio         Utah           2017-11-01 06:00
  </code></pre>
  
  <p>so basically for each distinct row I want to return 3 time periods starting from today,s date.
  I have the logic to get the datetime but I'm doing this through a loop of all the records which is a time consuming since my data would contain more than 1M rows . I was wondering is there a way to do this using the apply function, which I'm assuming it will be much faster.</p>