Custom conversion methods between String and Quantities were missing

de.hftstuttgart.cityunits.model/model/Quantities.genmodel

@@ -7,7 +7,9 @@
   <foreignModel>Quantities.ecore</foreignModel>
   <genPackages prefix="Quantities" basePackage="de.hftstuttgart.cityunits.model" disposableProviderFactory="true"
       ecorePackage="Quantities.ecore#/">
-    <genDataTypes ecoreDataType="Quantities.ecore#//QuantityDouble"/>
-    <genDataTypes ecoreDataType="Quantities.ecore#//QuantityLong"/>
+    <genDataTypes ecoreDataType="Quantities.ecore#//QuantityDouble" create="return de.hftstuttgart.units.NullableQuantity.create(it);"
+        convert="return it == null ? null : it.toString();"/>
+    <genDataTypes ecoreDataType="Quantities.ecore#//QuantityLong" create="return de.hftstuttgart.units.NullableQuantity.create(it);"
+        convert="return it == null ? null : it.toString();"/>
   </genPackages>
 </genmodel:GenModel>