AbstractGradeRecord is the abstract base class for Grade Records, which are
records of instructors (or the application, in the case of autocalculated
gradebooks) assigning a grade to a student for a particular GradableObject.
Calculate the mean course grade (whether entered or calulated) as a
percentage for all enrollments, leaving
students who've explicitly been given non-percentage-valued manual-only
course grades (such as "I" for incomplete) or null scores out of the calculation.
This method will fail unless this course grade was fetched "with statistics",
since it relies on having the total number of points possible available to
calculate the percentage.
Because the Gradebook now supports non-calculated manual-only grades with
no percentage equivalent, it is possible for the list of grades to include
codes that are not included in the defaultBottomPercents map.
This method is called by the Course Grade UI after gathering filtered enrollment
records and course grade records, and before actually formatting and downloading
the XLS or CSV file.