title: swift 3 convert string to objectID
body: <p>i work with swift 3 and nsoutlineview.
  i would like to save the objectID of a core data record into a textfield.
  so i have to convert it into a string:</p>
  
  <pre><code>txtFied.stringValue = "\(CoreData[outlineView.selectedRow].objectID)"
  </code></pre>
  
  <p>how can i convert it back to an NSManagedObjectID?</p>