Modify GradingPeriod Key

Summary of outcomes for Data Standard 3.0

• Problem Addressed: Prior to v3.0, the GradingPeriod key included the BeginDate, contributing to some risk of volatility in the model, as grading period dates may be subject to change.
• High-level Reasoning: While the GradingPeriod BeginDate may change, the PeriodSequence and SchoolYear are very unlikely to change. By removing the BeginDate from the key and adding the SchoolYear and Period Sequence, the key is much less volatile while still uniquely identifying each record as intended.
• Changes Made: Remove BeginDate from and add SchoolYear and PeriodSequence to the GradingPeriod key. BeginDate continues to be required (though not part of the identity).

Original ticket description follows:

Remove BeginDate from the GradingPeriod key and add SchoolYear and PeriodSequence to the key.