Change WorkletPointNeighborhood to specify neighborhood at runtime

Previously, WorkletPointNeighborhood had a template argument to select the size of the neighborhood. This change removes that template argument. Instead, the vtkm::exec::arg::BoundaryState methods now take in a size parameter when determining when it overlaps the boundary.

If in the future we want to add the ability to select the neighborhood size at compile-time (for performance reasons), I suggest adding this template argument to the OnBoundary tag for ExecutionSignature.