Did a bit of renaming of the support classes used for
WorkletPointNeighborhood. First, the OnBoundary tag is changed to
Boundary to match other tags and reflect some changes in the resulting
methods. Also moved the BoundaryState and Neighborhood classes from
vtkm::exec::arg to vtkm::exec to be more accessible. Finally, the
Neighborhood class name was changed to FieldNeighborhood to be more
specific on what role this class plays with neighborhood.
