Support ply files with normals

[weitsang]

Currently, vv can read PLY files without normal as attributes. Sometimes it is useful to precompute the normal of each point and store them in PLY files. The normals can be used by a subsequent subcommand.

[YoHoSo]

The sample file you gave me previously has the following header:

Since the header has

property float x
property float y
property float z

so currently vv/vvplay can read the normal files without problems.

Do you want me to add a subcommand to compute the normals?

[weitsang]

I believe the fields nx, ny, and nz are the normals. Currently they are not stored. We do not have a data structure to store them, I believe.

@wweqg is working on normal computations.

[YoHoSo]

Bo Kuan has mentioned about not having a data structure to store normals before. I think he is working on this. Maybe we should talk with him first so that I can avoid doing the same thing.