Multi-year cumulative GPA

[psvenk]

After implementing #74 and #99, add a feature which would use data from multiple years (some of which may be from imported data) to calculate a cumulative GPA.

[MohanMaker]

From #287: "The concept of "cumulative GPA" is confusing in the previous year: if it works properly, it shows the average GPA for that year; however, the wording seems to suggest that it shows the cumulative GPA for the past two years, or for all four years. Maybe we can call it "yearlong GPA" or something similar?"