Figure out how to handle TPDM Survey takers as the Survey domain becomes part of core

The survey domain allows for many different entities to take a survey, these include:

Student, Parent, Staff, Ed-Org from Ed-Fi core

Teacher Candidate, Applicant, Prospect from TPDM

The way this is currently modeled, in MetaEd we use a choice object (reference attached) to represent all of the above entities. This has the effect of adding a column for each type of entity to the SurveyResponse table with foreign keys out to each entitty table. (table reference attached)

This works fine in TPDM because TPDM can have core references in addition to TPDM references. Once we move Survey into core, the SurveyResponse entity will no longer be able to have references to TPDM entities. 

We need to find a way to model the takers of a survey in a way that doesn't place them on the table itself (at least for TPDM entites). One possible way of dealing with this is an associations between the TPDM "people" and the surveyresponse entity. The downside to this solution is has the potential to lead to multiple entities associated with a single survey response, which is not ideal.