decreased visibility of group-fields -> adapted tests and GroupService

src/main/java/mops/gruppen2/domain/Aggregate.java

@@ -1,15 +1,23 @@
 package mops.gruppen2.domain;
 
+import lombok.Getter;
 import mops.gruppen2.domain.event.Event;
 
 import java.lang.reflect.Method;
 
 public abstract class Aggregate {
 
+    @Getter
+    protected final long id;
+
+    protected Aggregate(long id) {
+        this.id = id;
+    }
+
     /**
      * Ruft die spezifische applyEvent-Methode im entsprechenden Aggregat auf.
      *
-     * @param event
+     * @param event Einzelne Änderung an dem Aggregat
      */
     public void applyEvent(Event event) {
         try {