Proposal: adding a `vtkPointCloud` data structure

When representing point-cloud as polydata, one has still have to create a cell-array for marking the verts. This can be quite expensive for large point clouds. Currently, a large slew of polydata filters and mappers simply do nothing (and rightly so) when no cell-array is specified.

This would be indeed be convenient, or at least less clunky, for handling lagrangian data. Nice one.