implemented task number 6
implemented DuplicateStudentException, is thrown when someone trys to add a
student which already exists in the StudentCatalog
Added Files:
src/java/de/fhworms/ztt/ox/campusdatarepository/DuplicateStudentException.java
src/java/de/fhworms/ztt/ox/campusdatarepository/StudentCatalog.java
src_test/java/unit/de/fhworms/ztt/ox/campusdatarepository/StudentCatalogTest.java
