Need to support ELL status and proficiency changes multiple time in a school year

The current data model does not support EL status and proficiency changes multiple times per year, as follows:

• StudentEducationOrganizationAssociation.LimitedEnglishProficiency (Descriptor) is not associated with a date and cannot hold multiple longitudinal statuses
• StudentLanguageInstructionProgramAssociation. EnglishLanguageProficiencyAssessment.Proficiency (Descriptor is part of a common type with only SchoolYear as a key.