MDL-9506 Removed the debugging message for the fetch() methods. Added droplow and...

MDL-9506 Removed the debugging message for the fetch() methods. Added droplow and keephigh rules to grade_category::aggregate_grades(). The only question now is what to do when the droplow or keephigh field is larger than the number of grades for a given user/item combination. At the moment it just gives that value 0.