Changing `vtkPointSet` to become a concrete class.
This commit implements cell handling pure virtual methods
inherited from vtkDataSet
. They either do nothing, return nullptr
,
or return 0. This change intents to give point clouds a proper data
structure. Point clouds should be instantiated using this new concrete
class. This change in the API follows this discussion:
https://discourse.vtk.org/t/proposal-adding-a-vtkpointcloud-data-structure/3872/
Addresses paraview/paraview#17185