Add better queries to vtkm::exec::arg::BoundaryState

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.