user cron job - kill reminder

[cataclysmic]

Check oTree instance for last worker action and send notification mail to user after long (48h) inactivity to close oTree.

• create local cronjob on user_add ~ 1.00am
• check if otree is active
  kill $(ps -axo user:20,pid,command | grep $USER | grep 'otree' | grep 'python' | awk '{if (NR==1) print $(2)}')
• query database for last user every 24h
• if last user activity > 48h, send reminder to experimenter to kill otree

Needs one script and one crontab entry.

User-specific cronjob:
https://askubuntu.com/questions/2368/how-do-i-set-up-a-cron-job#2369