If TVTK is frozen, paths to TVTK_classes are broken

As stated in the above, when a project using TVTK is frozen using cx_Freeze, all the .py files are zipped into a zipfile. TVTK then tries to find the zipfile within the zipfile which is a failure. Ideally, TVTK should check the `sys.executable` path if `sys.frozen` is defined.