Add CompositeCollection serialization

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/datastructure/CompositeCollection.java

@@ -2,6 +2,7 @@ package de.hft.stuttgart.citydoctor2.datastructure;
 
 
 import java.io.Serial;
+import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -9,7 +10,7 @@ import java.util.List;
  *
  * @author Riegel
  */
-public final class CompositeCollection {
+public final class CompositeCollection implements Serializable {
 
     @Serial
     private static final long serialVersionUID = -1867197873443341287L;