Implement changes to StudentSchoolAssociation

Implement the recommended changes identified in DATASTD-1939:

• Changes in MetaEd resulting in the Handbook documentation
• Update UML
• Update sample data and descriptor values as required.

Recommended Changes
Changes to StudentSchoolAssociation:

• Add attribute NextYearSchool, reference to School - The anticipated school of enrollment for the student for the next school year, possibly reflecting a rollover to the same school, the promotion to a feeder school, or an anticipated transfer.
• Add attribute NextYearGradeLevel, GradeLevel descriptor - The anticipate grade level for the student for the next school year.
• Deprecate attribute ("Will be removed in Data Standard v7.0") SchoolChoiceTransfer.
• Add attribute SchoolChoice, boolean - An indication of whether the student enrolled in this school under the provisions for public school choice.
• Add attribute SchoolChoiceBasis, descriptor - The legal basis for the school choice enrollment according to local, state or federal policy or regulation. (The descriptor provides the list of available bases specific to the state)
• Add attribute EnrollmentType, descriptor - The type of enrollment reflected by the StudentSchoolAssociation.