switch to getTenderReferencedDocument analogue to getContractReferencedDocument, added DatedReference class and setters/getters for Invoice

library/src/test/java/org/mustangproject/ZUGFeRD/ZF2EdgeTest.java

@@ -145,7 +145,7 @@ public class ZF2EdgeTest extends MustangReaderTestCase {
 	}
 
 	@Override
-	public IDatedReference getTenderReference() {
+	public IDatedReference getTenderReferencedDocument() {
 		return new IDatedReference() {
 			@Override
 			public String getID() {
@@ -159,6 +159,7 @@ public class ZF2EdgeTest extends MustangReaderTestCase {
 				} catch (ParseException e) {
 					// wont happen, I promise :-)
 				}
+				return null; // wont happen either
 			}
 		};
 	}

library/src/test/java/org/mustangproject/ZUGFeRD/ZF2PushTest.java

@@ -596,6 +596,7 @@ public class ZF2PushTest extends TestCase {
 			try {
 				SchemedID gtin = new SchemedID("0160", "2001015001325");
 				SchemedID gln = new SchemedID("0088", "4304171000002");
+				DatedReference dr=new DatedReference("90-kl-98798-C", sdf.parse("2025-10-12"));
 				ze.setTransaction(new Invoice().setCurrency("CHF").addNote("document level 1/2").addNote("document level 2/2").setDueDate(new Date()).setIssueDate(new Date()).setDeliveryDate(new Date()).setPaymentReference("Verwendungszweck").setDocumentName("Rechnung")
 					.setSellerOrderReferencedDocumentID("9384").setBuyerOrderReferencedDocumentID("28934")
 					.setDetailedDeliveryPeriod(new SimpleDateFormat("yyyyMMdd").parse(occurrenceFrom), new SimpleDateFormat("yyyyMMdd").parse(occurrenceTo))
@@ -608,6 +609,7 @@ public class ZF2PushTest extends TestCase {
 					.addCharge(new Charge(new BigDecimal(0.5)).setReason("quick delivery charge").setTaxPercent(new BigDecimal(16)))
 					.addAllowance(new Allowance(new BigDecimal(0.2)).setReason("discount").setTaxPercent(new BigDecimal(16)))
 					.addCashDiscount(new CashDiscount(new BigDecimal(2), 14))
+					.setTenderReferencedDocument(dr)
 					.setDeliveryDate(sdf.parse("2020-11-02")).setNumber(number).setVATDueDateTypeCode(EventTimeCodeTypeConstants.PAYMENT_DATE)
 					.setInvoiceReferencedDocumentID("abc123").addInvoiceReferencedDocument(new ReferencedDocument("abcd1234"))
 				);