Defines a relation between a submission and the submission's submitters.
- A submitter can have its own grade separate from the grade of the submission,
useful in providing user with different grades in group submissions.
- A submitter can have its own feedback separate from the feedback of the submission,
useful when different feedback is needed in group submissions
- submission and submitter are unique,
meaning a user can't be a submitter more than once on a submission.
Notice that equals and hashcode also reflect this relationship.