minor code improvements

src/main/java/it/unibz/inf/isochrone/config/ConfigDataset.java

@@ -55,7 +55,8 @@ public class ConfigDataset {
 
 		@Override
 		public Calendar transform(final String str) {
-			DateFormat dateFormat = new SimpleDateFormat(DATE_FORMAT);
+			final DateFormat dateFormat = new SimpleDateFormat(DATE_FORMAT);
+
 			Calendar targetTime = Calendar.getInstance();
 			try {
 				targetTime.setTimeInMillis(dateFormat.parse(str).getTime());