title: Groupby - Generate date time as sequence
body: <p>I have a dataframe as shown below</p>
  
  <pre><code>session      slot_num
  s1           1
  s1           2
  s1           3
  s1           3
  s1           4
  s1           4
  s2           1
  s2           2
  s2           2
  s2           3
  s2           4
  s2           5
  s2           5
  s2           6
  s3           1
  s3           1
  s3           2
  s3           3
  </code></pre>
  
  <p>from the above I would like to create a column appt_time as shown below.</p>
  
  <p>Expected output</p>
  
  <pre><code>session      slot_num     appt_time
  s1           1            2020-01-06 09:00:00
  s1           2            2020-01-06 09:20:00
  s1           3            2020-01-06 09:40:00
  s1           3            2020-01-06 09:40:00
  s1           4            2020-01-06 10:00:00
  s1           4            2020-01-06 10:00:00
  s2           1            2020-01-06 08:20:00
  s2           2            2020-01-06 08:40:00
  s2           2            2020-01-06 08:40:00
  s2           3            2020-01-06 09:00:00
  s2           4            2020-01-06 09:20:00
  s2           5            2020-01-06 09:40:00
  s2           5            2020-01-06 09:40:00
  s2           6            2020-01-06 10:00:00
  s3           1            2020-01-09 13:00:00
  s3           1            2020-01-09 13:00:00
  s3           2            2020-01-09 13:20:00
  s3           3            2020-01-09 13:40:00
  </code></pre>
  
  <p>Explanation:
  for session = s1, appt_start time = 2020-01-06 09:00:00, then for each increase in slot_num for that session increment appt_time by 20 minutes.</p>
  
  <p>for session = s2, appt_start time = 2020-01-06 08:20:00, then for each increase in slot_num for that session increment appt_time by 20 minutes.</p>
  
  <p>for session = s3, appt_start time = 2020-01-09 13:00:00, then for each increase in slot_num for that session increment appt_time by 20 minutes.</p>