start to add ability to merge tunes

started refactoring tune builders to use string type hint instead of StringAtom

src/Domain/Core/Tune.php

@@ -124,6 +124,7 @@ class Tune implements EntityInterface
     /**
      * @param string $title
      * @return $this
+     * @deprecated
      */
     public function addTitle(string $title) : Tune
     {
@@ -278,15 +279,19 @@ class Tune implements EntityInterface
                 call_user_func([$this, 'set'.$attribute], $value);
                 break;
 
+            case 'Authors':
+                call_user_func([$this, 'addAuthor'], $value);
+                break;
+
             default:
                 $this->append($attribute, $value);
         }
     }
 
-    protected function append($attribute, array $values)
+    protected function append($attribute, MultivalueAttribute $values)
     {
         $method = 'add' . substr($attribute, 0, -1);
-        foreach ($values as $value) {
+        foreach ($values->toArray() as $value) {
             call_user_func([$this, $method], $value);
         }
     }
@@ -329,13 +334,18 @@ class Tune implements EntityInterface
      * @param StringAtom $musicLine
      * @return $this
      */
-    public function appendMusicLine(string $musicLine)
+    public function addMusicLine(string $musicLine)
     {
-        $this->music .= $musicLine->getValue() . PHP_EOL;
+        $this->music .= $musicLine . PHP_EOL;
 
         return $this;
     }
 
+    public function getMusic()
+    {
+        return $this->music;
+    }
+
     public function merge(Tune $source)
     {
         //TODO