* Correctly use the VRKey for all vr variables... instead of TagName

   * Correctly use the VRKey for all vr variables... instead of TagName
     or std::string
   * Replace the definition of VRKey from std::string to a class
     containing a char[2]. All methods are inline to have no speed low
   * Create the GDCM_VRUNKNOWN = "  " for the VR values
   -- BeNours