-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

# Conflicts:
#   .idea/dbnavigator.xml
#   .idea/misc.xml
#   .idea/uiDesigner.xml
#   pom.xml

Jpa

See merge request !2

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'

# Conflicts:
#	src/main/java/com/example/gui/CourseModificationController.java
#	src/main/java/com/example/gui/CoursesOfStudyModificationController.java
#	src/main/java/com/example/gui/DegreeDetailsController.java
#	src/main/java/com/example/gui/MainController.java
#	src/main/java/com/example/gui/StudentModificationController.java
#	src/main/resources/com/example/gui/course-modification.fxml
#	src/main/resources/com/example/gui/courses-of-study-modification.fxml
#	src/main/resources/com/example/gui/main.fxml
#	src/main/resources/com/example/gui/student-modification.fxml

Gui-V1

See merge request !1