make grade level optional in Assessments model

NWEA tests generally span a large range of grades or are not even associated with a grade-level (by design). We do have suggested grades, but we don’t restrict tests for students by their grade.

The Ed-Fi Assessment model currently requires the assessedGradeLevelDescriptor attribute. Currently, we plan to use the value of "Ungraded" for this attribute, but we think it may make more sense to make the assessedGradeLevelDescriptor attribute optional.

Here are some example test names for context:

• "Growth: Reading K-2 AL 2013"
• "Growth: General Science AK 2006 V2"
• "Growth: Reading 6+ OK 2016"
• "Growth: Language 2-12 OK 2016"