bug fix no duplicate (multiple) courses in CourseModificationController + CoursesOfStudyDetailsController anymore

-add toString() for Course, Student and CoursesOfStudy
-fix null-pointer-exception in detailControllers + SwitchWindowHelper
-add CoursesOfStudy Column in Student
-add find-Methods to JpaService
-connect Gui to DB

passing values to controllers is possible with static methods +
added getters + clean up