Add controlled list of research status (for now for actors)

src/MDResearchStatusSet.php

@@ -0,0 +1,23 @@
+<?PHP
+/**
+ * Research status.
+ *
+ * @see https://tools.ietf.org/html/rfc2426
+ * @see https://tools.ietf.org/html/rfc6350
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+/**
+ * Class containing available place types.
+ */
+final class MDResearchStatusSet extends MDValueSet {
+
+    const RESEARCH_STATUS = [
+        'bad',
+        'medium',
+        'good',
+    ];
+
+}