add ExtractTune use case

src/Domain/Core/Tune.php

@@ -80,6 +80,20 @@ class Tune implements EntityInterface
         }
     }
 
+    public function getETag()
+    {
+        return md5(implode(';', array_flatten([
+            $this->getTitles(),
+            $this->getAuthors(),
+            $this->getComposers(),
+            $this->getGroups(),
+            $this->getHistory(),
+            $this->getOrigins(),
+            $this->getRhythms(),
+            //TODO: add settings
+        ])));
+    }
+
     /**
      * @param UnsignedIntegerAtom $index
      * @return $this