support ply texcoord

[HaruoOgawa]

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[ondys]

while adding texcoord support to ply decoding would be a nice feature to have, I'm not sure if the code in this PR would actually work for most cases. It would certainly not work for texture coordinates encoded with our PlyEncoder which would be probably the main requirement.
The most common way to store texcoord in ply is by using an face element list property, something like:

element face numFaces
...
property list uint8 float texcoord

Is there any software that produces per-vertex texcoords with texture_u and texture_v properties as proposed in this PR?