The email grouping strategy can be defined at different levels (per wiki, and per user) and is specific for a given scheduler, using a new dedicated XObject XWiki.Notifications.Code.NotificationEmailGroupingStrategyPreferenceClass.
So an admin can add one or severals xobjects in XWiki.XWikiPreferences for defining the different strategies for each scheduler, at wiki level: the xobject needs to contain the name of the strategy to use, and the name of scheduler (hourly, daily, weekly or live).
This value can be overridden by a user if they perform adding the same type of xobject in their own user profile.
