Refactor: Unify and generalize CityGml3FeatureMapper

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

@@ -9,18 +9,17 @@ public class TunnelPart extends AbstractTunnel {
 
     private Tunnel parent;
 
-    private TunnelPart() {
-    }
-
-    public TunnelPart(Tunnel parent) {
-        this.parent = parent;
-        parent.addTunnelPart(this);
+    public TunnelPart() {
     }
 
     public Tunnel getParent() {
         return parent;
     }
 
+    public void setParent(Tunnel parent) {
+        this.parent = parent;
+    }
+
     @Override
     public CityObject getTopLevelCityObject() {
         return getParent().getTopLevelCityObject();