Refactor: Extract and rename methods.

Rename GradedCurriculum#log to GradedCurriculum#report.
Merge 'untaken_courses' and 'failed_courses' as 'incomplete'.
Add Course#passed.
Add GradedCurriculum#is_completed
Rename GradedCurriculum#letterToGrade to Course#numberGrade.
Optimize gpa calculation.