add. some validation and opts to add_cohort_data.
mod. Default request project to the one stored in usertags_meta.
mod. get_user to utilize query_calls for retrieving cohort users and id
mod. handle metrics call failures in agg and raw requests.
mod. Use a generator to return cohort IDs.  They may be fairly large and the option not to store the entire set in memory is useful.
rm. JOB_STATUS enum, deprecated.
rm. method to retrieve cohort users.  use method in query subpackage - get_cohort_users.