+ Use the new resource/operator framework to load model files.
+ Check more pointers to avoid crashes.
+ Eliminate warning from vtkModelMultiBlockSource about model being unset
   because now each **resource** should have its own representation,
  so we should always render everything the resource has to offer
  rather than trying to traverse just one model.
+ Set up SMTK model representation properly: set both the resource and the
  vtkSMTKWrapper instance so that selections are properly synchronized.