RFC: COMP: use relative includes for VTK and ITK

Isaiah requested to merge github/fork/ihnorton/vtkitk_relative_includes into master

Opening for discussion, ref https://discourse.slicer.org/t/duplicated-includes/1862

:s5 $ cat `find ./ -name *.cxx -or -name *.h` | egrep "#include.*itk" | less
:s5 $ cat `find ./ -name *.cxx -or -name *.h` | egrep "#include.*vtk" | less
  • Build system TODO
    • Teach build system to clone {vtk,itk} in e.g. SuperBuild/VTKv9/vtk
    • Teach build system to pass single include path for vtk as e.g. SuperBuild/VTKv9/
    • ? Fix bundling cmakescript ?

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