CLC-2816 GPA calc now uses historical cumulative GPA

GPA calc now uses historical cumulative GPA as the basis for its calculations. 

Fixes https://jira.ets.berkeley.edu/jira/browse/CLC-2816 as well as https://jira.ets.berkeley.edu/jira/browse/CLC-3041 and https://jira.ets.berkeley.edu/jira/browse/CLC-3026