[vtkusers] loop over boundary vertices

[Nico Schlömer]

Hi everyone,

I'd like to loop over all boundary vertices in a vtkDataSet. I see
that feature edges have `BoundaryEdgesOn()`, but this seems a little
wasteful since 2D I'd visit every boundary vertex twice, and even more
times in 3D.

Is there a better way for looping over the boundary nodes?

Cheers,
Nico