Previously, vtkm::exec::arg::BoundaryState only provided methods that
said whether or not the neighborhood extened past the boundary of a
mesh. That is fine for a 3x3x3 neighborhood, which can only extend over
the boundary by one. However, that is problematic for larger
neighborhoods where you may need to know how far neighborhood extends
over the boundary.

This changes allows you to query how far the neighborhood extends within
the constrains of the boundary.