Use constants for accessing the properties of an subject

Libs/XNAT/Core/ctkXnatSubject.cpp

@@ -119,10 +119,10 @@ void ctkXnatSubject::fetchImpl()
 
   foreach (ctkXnatObject* experiment, experiments)
   {
-    QString label = experiment->property ("label");
+    QString label = experiment->property (ctkXnatObjectFields::LABEL);
     if (!label.isEmpty())
     {
-      experiment->setProperty ("id", label);
+      experiment->setProperty (ctkXnatObjectFields::LABEL, label);
     }
 
     this->add(experiment);

Libs/XNAT/Core/ctkXnatSubject.h

@@ -46,9 +46,13 @@ public:
   QList<ctkXnatProject*> getProjects() const;
 
   const QString& insertDate() const;
+
+  /// @warning does not overwrite the insert date on the server, since this is read only
   void setInsertDate(const QString& insertDate);
 
   const QString& insertUser() const;
+
+  /// @warning does not overwrite the insert user on the server, since this is read only
   void setInsertUser(const QString& insertUser);
 
   virtual QString resourceUri() const;