-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

add enums to combo boxes, remove 'setComboBox'-Method in MainController because it works only with Strings and again some renaming of old 'degreeModification' to 'coursesOfStudyModification'