Proposal: adding a `vtkPointCloud` data structure

[lassoan (Andras Lasso)]

@Yohann_Bearzi

If a poly data can be a point cloud as well as a triangular mesh, how do we grey out unapplicable algorithms?

It is an application implementation detail. There are certainly many options that are as good as checking the class of an object.

@Yohann_Bearzi

Do we just allow them an do not produce anything when there is no vtkVertex cells?

The obvious choice would be to use all points of the vtkPolyData if there are no vtkVertex cells.

@Michael

To some extent, having a dedicated data structure for each simplex set is useful. Point cloud (0D) of course but lines-only (1D), triangles-only (2D) and tetrahedron-only (3D) data structures can be very useful as well.

Algorithm developers tend to prefer fine-grained class hierarchy (data structures and filters with narrow scope), while it makes applications simpler if there are less types (and they have broader scope). VTK so far hit a good balance and it would be nice to preserve this.