build_octree chokes on unknown point properties

[SirVer]

A File like this:

ply
format binary_little_endian 1.0
element vertex 38400000
property float x
property float y
property float z
property uchar red
property uchar green
property uchar blue
property uchar alpha
element face 0
property list uchar int vertex_indices
end_header

triggers this error:

thread 'main' panicked at 'Unknown property 'alpha' on 'vertex'.', src/ply.rs:339
note: Run with `RUST_BACKTRACE=1` for a backtrace.

solution is to skip over unknown properties while parsing the PLY. The framework for this is in place, it just needs to be done.