There are occasions when you need a worklet to opeate on 2D or 3D
indices. Most worklets operate on 1D indices, which requires recomputing
the 3D index in each worklet instance. A workaround is to use a worklet
that does a 3D scheduling and pull the working index from that.
The problem was that there was no easy way to get this 3D index. To
provide this option, a feature was added to the `BoundaryState` class
that can be provided by `WorkletPointNeighborhood`.
Thus, to get a 3D index in a worklet, use the
`WorkletPointNeighborhood`, add `Boundary` as an argument to the
`ExecutionSignature`, and then call `GetCenterIndex` on the
`BoundaryState` object passed to the worklet operator.
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.
