[Paraview] Clip filter doesn't operate on points

[Ken Olum]

I created a file with a set of points in an unstructured_grid.  Glyph
was happy to display glyphs of the locations of the points.  But when I
wanted to select a subset of the points with Clip, it never passed on
any of them.  Apparently Clip doesn't work with points, but only with
cells.  When I made a vertex at each point, it worked fine.

I can't see any reason why Clip should not work with points, but if
there is at least it should be documented.

Thanks.

                                        Ken