Credential.GradeLevel should be a collection and perhaps made optional

In Data Standard 2.1 the domain entity Credential has a required GradeLevel. However, I believe credentials are often provided that cover multiple grade levels.

Second, it might be preferable to make this optional to expand the applicability of this entity. For example, I am thinking of credentials that may cover librarianship or similar roles that are instructional but may not be indexed to specific grade levels.

It is worth noting for comparison purposes that StaffSchoolAssociation.GradeLevel is an optional collection.