Add base support for cohorted group configurations

TNL-649

Introduced a new scheme field to user partitions that controls how a user gets assigned
to one of the partitions groups. The default scheme is random, but the next step will be
to introduce a new scheme that will assign the user based upon the cohort they belong to.
I needed it in the constructor for all the tests that create partitions directly. Maybe I should just say that there is a true default, and remove the errors as you suggested initially.