Change GradingPeriod key to make non-volatile

GradingPeriod contains a BeginDate as part of the natural key. As with most dates this entity is subject to modification and likely more prone to error in data entry than with other fields.

Vendors have reported concerns that shifts in school calendars - perhaps retroactively (e.g. a state issuing a mandate that a snow day or other event should result in a retroactive shift to school calendars) could result in significant issues with updating already-reported entities that rely on GradingPeriod, such as Grades.

This entity seems to be a candidate for the partial key surrogate pattern, perhaps even by relying on the existing CodeValue element.