clean up + rename 'DegreeModification' to 'CoursesOfStudyModification'

.../thdeg/grademanager/gui/CourseModificationController.java

src/main/java/de/thdeg/grademanager/gui/CoursesOfStudy.java

...rademanager/gui/CoursesOfStudyModificationController.java

...va/de/thdeg/grademanager/gui/DegreeDetailsController.java

src/main/java/de/thdeg/grademanager/gui/Main.java

src/main/java/de/thdeg/grademanager/gui/MainController.java

...thdeg/grademanager/gui/StudentModificationController.java

src/main/java/de/thdeg/grademanager/model/Calcuable.java

src/main/java/de/thdeg/grademanager/model/Course.java

...main/java/de/thdeg/grademanager/model/CoursesOfStudy.java

src/main/java/de/thdeg/grademanager/model/Enrollment.java

src/main/java/de/thdeg/grademanager/model/Lecturer.java

src/main/java/de/thdeg/grademanager/model/Main.java

src/main/java/de/thdeg/grademanager/model/Student.java

...in/java/de/thdeg/grademanager/model/UniversityMember.java

.../thdeg/grademanager/model/enumeration/BachelorDegree.java

...hdeg/grademanager/model/enumeration/BachelorSemester.java

...a/de/thdeg/grademanager/model/enumeration/CourseType.java

...ava/de/thdeg/grademanager/model/enumeration/ExamType.java

...de/thdeg/grademanager/model/enumeration/FieldOfStudy.java