Add field for tag GND IDs

Close #41

src/CsvxmlAvailableFields.php

@@ -991,6 +991,10 @@ public function __construct(string $lang) {
                 dependsOn: ["tag" . $i],
                 name_human_readable: $tlLoader->tl("overview", "overview", 'tags') . " (Wikidata)",
             );
+            $availableFields[$tlLoader->tl("overview", "overview", 'tags')]["tag_gnd" . $i] = new FieldEntry(
+                dependsOn: ["tag" . $i],
+                name_human_readable: $tlLoader->tl("overview", "overview", 'tags') . " (GND)",
+            );
 
         }