Create the service stub for peer grading

This stub just has some doc and empty functions so we can discuss if the contract is reasonable, then potentially build on this frame.

peer_grading/peer_grading_service.py

+
+
+def get_next_submission(student_id, location):
+    """
+    Request the next submission to be peer graded.
+    @param student_id: The student requesting to grade a peer. Must check to determine if the requesting student has
+                       submitted an answer of their own.
+    @param location: The associated location for the submission to be graded.
+    @return: The submission to grade, if one is available.
+    """
+    pass
+
+
+def get_last_submission(student_id, location):
+    """
+    Used to give visibility to scoring and workflow for a submission in peer grading. If the student has submitted a
+    submission and has graded enough peers, this function will return the submission as is, with all available scoring
+    data. If the student has not finished grading peers, scoring information on their submission is withheld.
+    @param student_id: The student.
+    @param location: The associated location.
+    @return: The student's latest submission, restrained on workflow completion.
+    """
+    pass
+
+
+def submit(submission):
+    """
+    Submit a submission for peer grading.
+    @param submission: The submission to add to the peer grading queue. Should contain the student_id,
+                       associated location, and all answer related fields prepopulated. Submission date,
+                       preferred grader, and other attributes can be determined internally.
+    @return: The saved submission.
+    """
+    pass
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