add GruppenService and EventRepository and Event object

src/main/java/mops/gruppen2/repositories/EventRepository.java

@@ -0,0 +1,7 @@
+package mops.gruppen2.repositories;
+
+import mops.gruppen2.DTO.Event;
+import org.springframework.data.repository.CrudRepository;
+
+public interface EventRepository extends CrudRepository<Event, Long> {
+}