Fixup VTK 7.x support, remove VTK 6.x support #50
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find_package without version would allow VTK 6.x with a missing
vtkftgl component, so specify a minimum version.
Remove VTK 6.x, as VTK 7.1.1 is the oldest supported version
from Kitware (see https://vtk.org/download/) and 7.1 is 5 years old.
Add missing vtkFiltersHybrid component, required for e.g.
vtkImplicitModeller::ComputeModelBounds.