Style: Make fields final

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/zip/CityGmlZipInputStream.java

@@ -8,8 +8,8 @@ import java.util.zip.ZipFile;
 
 public class CityGmlZipInputStream implements AutoCloseable {
 
-    private ZipFile zip;
-    private ZipEntry zipEntry;
+    private final ZipFile zip;
+    private final ZipEntry zipEntry;
     private boolean closed = false;
     private static final long MB = 1024 * 1024L;