First step in the removal of VTK 4 compatibility layer. I removed
vtkSource, vtkProcessObject and all of their subclasses. All algorithms
should be subclasses of vtkAlgorithm now.

Also removed vtkPLOT3DReader. This reader was using legacy API. Use
vtkMultiBlockPLOT3DReader instead

Made changes vtkSpatialRepresentationFilter that are not backwards
compatible. This filter now produces a multi-block output and which
levels are computed is controlled using the AddLevel method.