Add GradePointAverageType to StudentAcademicRecord.GradePointAverage

Summary of outcomes for Data Standard 3.2

• Problem Addressed: Grade point average can be cut many different ways and the initial model to have GPA defined by a weighted system, cumulative boolean, and value are not sufficient to cover all use cases.
• High-level Reasoning: Based on feedback from the Course Transcript SIG, a more generic GradePointAverageType (rather than weighted system descriptor) would be a better identifier for locally-defined types of GPAs. The cumulative boolean can still add information but should not be required or a part of the key. Also, an optional Max value for a GPA would be helpful in defining the scale.
• Changes Made: Renamed GradePointAverageWeightedSystem descriptor to GradePointAverageType. Updated cardinality of IsCumulative to optional. Added MaxGradePointAverageValue as an optional element. Tweaked the definitions to match expected use cases. Deprecated GradeValueQualifier on StudentAcademicRecord.

Original ticket description follows:

From the Course Transcript SIG, there are far more kinds of grade point averages than the existing model (v3.2-a) will allow. A generic GradePointAverageType descriptor would allow systems and implementations to define custom grade point averages to fit the need.

The Identity for GradePointAverage would become just the GradePointAverageType descriptor (remove GradePointAverageWeightedSystem descriptor and IsCumulative boolean from identity).

A definition for the new descriptor as well as a value set need to be defined.