Change Assessment entity natural key to make it non-volatile

Summary of outcomes for Data Standard 2.1

• Problem Addressed: AcademicSubject and AssessedGradeLevel are not relevant for some assessments. However, these fields are required as part of the key.
• High-level Reasoning: By adding a "Cross Subject" enumeration to AcademicSubject and a "No grade level" enumeration to GradeLevel, these assessments may still be accommodated with less impact than removing these fields from the key and making them optional or allowing null values in the key. Larger model changes may be considered in the future.
• Changes Made: Added enumeration "Cross Subject" to AcademicSubjectMap type and enumeration "No grade level" to GradeLevelMap type and added both enumerations to the sample xml files for AcademicSubjectDescriptor and GradeLevelDescriptor

Original ticket description follows:

The Assessment entity natural key is volatile due to its reliance on several volatile sub-fields. The Title of an assessment instrument is subject to change over time, and arguably the AcademicSubject and AssessedGradeLevel are as well. There may also be cases where the AssessedGradeLevel is not seen as relevant (i.e. a single instrument that is applied to several grade levels).

As a result of this volatility, a system that attempts to use this key in data exchange over time is likely to break.

This entity is a candidate for the partial key surrogate pattern.