Refactor: Make GenericAttribute serializable

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

@@ -5,8 +5,14 @@ import org.apache.logging.log4j.Logger;
 import org.citygml4j.core.model.core.AbstractGenericAttributeProperty;
 import org.citygml4j.core.model.generics.*;
 
+import java.io.Serial;
+import java.io.Serializable;
 
-public class GenericAttribute {
+
+public class GenericAttribute implements Serializable {
+
+    @Serial
+    private static final long serialVersionUID = -4101408947632132328L;
 
     private static final Logger logger = LogManager.getLogger(GenericAttribute.class);