Extend importer

Importer reads BillingSpecifiedPeriod of ApplicableHeaderTradeSettlement
and SpecifiedLineTradeSettlement.
Add ZugferdImporter#getDetailedDeliveryPeriodFrom and
ZugferdImporter#getDetailedDeliveryPeriodTo.
Fix extractIssuerAssignedID.

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

@@ -0,0 +1,43 @@
+package org.mustangproject.ZUGFeRD;
+
+import static org.junit.Assert.assertEquals;
+
+import java.io.IOException;
+import java.nio.charset.Charset;
+import java.text.ParseException;
+import java.util.Date;
+
+import org.junit.Test;
+
+public class InvoicingPeriodTestCase {
+  @Test
+  public void readBillingSpecification() throws IOException {
+    final String file = ResourceUtilities.readFile(Charset.defaultCharset(),
+        "src/test/resources/factur-x_invoicingPeriod.xml");
+    final ZUGFeRDImporter zi = new XRechnungImporter(file.getBytes());
+
+    // Reading ZUGFeRD
+    assertEquals(date("20220829"), zi.getLineItemList().get(0).getDetailedDeliveryPeriodFrom());
+    assertEquals(date("20220831"), zi.getLineItemList().get(0).getDetailedDeliveryPeriodTo());
+    assertEquals(date("20220901"), zi.getLineItemList().get(1).getDetailedDeliveryPeriodFrom());
+    assertEquals(date("20220902"), zi.getLineItemList().get(1).getDetailedDeliveryPeriodTo());
+    assertEquals(null, zi.getLineItemList().get(2).getDetailedDeliveryPeriodFrom());
+    assertEquals(date("20220909"), zi.getLineItemList().get(2).getDetailedDeliveryPeriodTo());
+    assertEquals(date("20220826"), zi.getDetailedDeliveryPeriodFrom());
+    assertEquals(date("20220902"), zi.getDetailedDeliveryPeriodTo());
+    //general asserts
+    assertEquals("1634.76", zi.getAmount());
+    assertEquals("RE1000", zi.getInvoiceID());
+    assertEquals("Sell", zi.getSellerTradePartyAddress().getCityName());
+    assertEquals("Beier", zi.getBuyerTradePartyName());
+  }
+  
+  private Date date(String toParse){
+    try {
+      return ZUGFeRDDateFormat.DATE.getFormatter().parse(toParse);
+    }
+    catch (final ParseException e) {
+      return null;
+    }
+  }
+}