Remove Grading Periods as a required collection for Session

Summary of outcomes for Data Standard 3.0

• Problem Addressed: Prior to v3.0, at least one GradingPeriod reference was required in order to create a Session. In reality, GradingPeriods may not be defined at the time when Sessions are defined, forcing implementations to prematurely create grading periods.
• High-level Reasoning: By making the GradingPeriod reference on Session an optional collection, implementations are not forced to create GradingPeriods in order to define a Session.
• Changes Made: The Session.GradingPeriod reference was changed to optional collection.

Original ticket description follows:
We are not sure as to why Grading Periods are part of the Natural Key of sessions. Depending on how a school decides to set up a schedule in their SIS it is possible that certain Sessions (or Terms) do not have a grading period associated with it.

This has become a big issue with trying to send Standards based grades as standards are typically not associated with a Grading Period. Elementary schools are now forced to create grading periods that are not an actual reflection of their grading set up to send sessions.