Extract version number from pom.xml to reduce duplication of the version number

src/main/java-templates/org/mustangproject/ZUGFeRD/Version.java

@@ -0,0 +1,5 @@
+package org.mustangproject.ZUGFeRD;
+
+public final class Version {
+    public static final String VERSION = "${project.version}";
+}

src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRDExporter.java

@@ -237,7 +237,6 @@ public class ZUGFeRDExporter implements Closeable {
 
 	// // MAIN CLASS
 	private PDFAConformanceLevel conformanceLevel = PDFAConformanceLevel.UNICODE;
-	private String versionStr = "1.4.1";
 
 	// BASIC, COMFORT etc - may be set from outside.
 	private ZUGFeRDConformanceLevel zUGFeRDConformanceLevel = ZUGFeRDConformanceLevel.EXTENDED;
@@ -466,7 +465,7 @@ public class ZUGFeRDExporter implements Closeable {
 			String creator, boolean attachZugferdHeaders) throws IOException,
 			TransformerException {
 		String fullProducer = producer + " (via mustangproject.org "
-				+ versionStr + ")";
+				+ Version.VERSION + ")";
 
 		PDDocumentCatalog cat = doc.getDocumentCatalog();
 		PDMetadata metadata = new PDMetadata(doc);

src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRDExporterFromA1Factory.java

@@ -29,8 +29,6 @@ import java.io.InputStream;
 import java.util.GregorianCalendar;
 
 public class ZUGFeRDExporterFromA1Factory {
-    private static final String VERSION_STR = "1.4.0";
-
     private boolean ignoreA1Errors = false;
     private ZUGFeRDConformanceLevel zugferdConformanceLevel = ZUGFeRDConformanceLevel.EXTENDED;
     private PDFAConformanceLevel conformanceLevel = PDFAConformanceLevel.U;
@@ -89,7 +87,7 @@ public class ZUGFeRDExporterFromA1Factory {
     }
 
     private void makePDFA3compliant(PDDocument doc) throws IOException {
-        String fullProducer = producer + " (via mustangproject.org " + VERSION_STR + ")";
+        String fullProducer = producer + " (via mustangproject.org " + Version.VERSION + ")";
 
         PDDocumentCatalog cat = doc.getDocumentCatalog();
         PDMetadata metadata = new PDMetadata(doc);