Allow Grouping of CommCareCases

There is something similar being used by reminders called SurveySample. However, if we plan on using CommCareCaseGroups for something other than reminders, then we'd want to make it more generic. 

It wasn't immediately obvious to me that a SurveySample was simply a group of CommCareCases at first, so best to avoid confusion for others later on. :-)

Migration and rename following shortly!