closes #172, fixes zero rated products, added some lines to the test of the XRechnung, have the XRechnung test actually wriote to file and have the XR test file validated automatically

library/src/main/java/org/mustangproject/ZUGFeRD/IZUGFeRDExportableItem.java

@@ -26,8 +26,6 @@ package org.mustangproject.ZUGFeRD;
  * @author jstaerk
  * */
 
-import org.mustangproject.ZUGFeRD.model.TaxCategoryCodeTypeConstants;
-
 import java.math.BigDecimal;
 
 public interface IZUGFeRDExportableItem extends IAbsoluteValueProvider{
@@ -74,10 +72,6 @@ public interface IZUGFeRDExportableItem extends IAbsoluteValueProvider{
 		return BigDecimal.ONE.setScale(4);
 	}
 
-	default String getCategoryCode() {
-		return TaxCategoryCodeTypeConstants.STANDARDRATE;
-	}
-
 	/***
 	 * the ID of an additionally referenced document for this item
 	 * @return the id as string

library/src/main/java/org/mustangproject/ZUGFeRD/IZUGFeRDExportableProduct.java

@@ -106,11 +106,12 @@ public interface IZUGFeRDExportableProduct {
 	}
 	
 	default String getTaxCategoryCode() {
-		 if (isIntraCommunitySupply()) {
+		if (getVATPercent().equals(new BigDecimal(0))) {
+			return "Z"; // zero rated goods
+		} else if (isIntraCommunitySupply()) {
 			 return "K";
 		 } else {
-			 return "S";
-			 
+			 return "S"; // one of the "standard" rates (not neccessarily a default rate, even a deducted VAT is standard calculation)
 		 }
 	 }
 	

library/src/main/java/org/mustangproject/ZUGFeRD/TransactionCalculator.java

@@ -147,7 +147,7 @@ public class TransactionCalculator implements IAbsoluteValueProvider {
 			BigDecimal percent = currentItem.getProduct().getVATPercent();
 			LineCalculator lc = new LineCalculator(currentItem);
 			VATAmount itemVATAmount = new VATAmount(lc.getItemTotalNetAmount(), lc.getItemTotalVATAmount(),
-					currentItem.getCategoryCode());
+					currentItem.getProduct().getTaxCategoryCode());
 			VATAmount current = hm.get(percent);
 			if (current == null) {
 				hm.put(percent, itemVATAmount);