Missing cube.ply from Matrix Transform tutorial

[taketwo]

In the Matrix Transform tutorial the example screenshots show rotation of a cube, which is loaded from a certain cube.ply file. This file is missing from the repo. Even though the example code can be run with any PLY and PCD file, of course, the lack of this file may cause some confusion.

Would be nice to add the file so that the user can see exactly same result as on the screenshots. And probably a usual note in the tutorial text with a link to the file. @VictorLamoine?