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.
I'm getting 
Unknown format code 'f' for object of type 'str'
here. Might be some check is needed before for users_grade_average['grade__avg'] ?