COMP: Update CMakeLists to support VTK 6

This commits updates the CMakeLists.txt of each module to explicitly
include SlicerExecutionModel and VTK6 project.

It also changes the FinslerBacktracing implementation to work with VTK 6
by using a vtkTrivialProducer.
BUG: Ensure Compare executable register ITK IO factories.
Merge pull request #3 from jcfr/add-vtkv6-itkv4-support

Add vtkv6 itkv4 support