Make consistent name for VTKSurfaceModel

[MattClarkson]

VTKSurfaceModel loads from file, so needs a proper absolute or relative pathname.
But vtk_surface_model_loader and vtk_surface_model_directory_loader end up with different results. The first uses .json which has a name field, that is independent from the file name. The second currently ends up with a basename with extension.

Solution:
The .json loader should use the name in the .json file, which wont have extension, or may not even look like a file name.
The directory loader should end up with the basename without extension.

In both cases, there is already an additional field called source_file which should store the filename anyway.

This seems consistent to me.