Add controlled list of data history subjects

Close #3

src/MDDataHistorySubjectsSet.php

@@ -0,0 +1,30 @@
+<?PHP
+/**
+ * Contains a class for controlling the list of available subjects for data history writing.
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+/**
+ * Class containing available options for data history subjects.
+ */
+final class MDDataHistorySubjectsSet extends MDValueSet {
+
+    const AVAILABLE_SUBJECTS = [
+        'appointment',
+        'collection',
+        'contact',
+        'exhibition',
+        'literature',
+        'loan',
+        'museum',
+        'object',
+        'podcast',
+        'podcast_episode',
+        'sammlung',
+        'series',
+        'user',
+    ];
+
+}