mattdrayer/api-org-course-avg: Added average grade to response

@dino-cikatic, added average grade to the response for /organizations/(id)/metrics -- note that the value represents the average grade across all users matching the query criteria.  So, if you filter by multiple course_id values (courses=course1,course2,course3) you will get the average grade across these three courses.  Filtering by a single value will provide you with the average for those organization users, for the specified course.