Support plain x/y/z/r/g/b point clouds in ply files

[mrxz]

Fixes #329

This PR implements point cloud decoding support in the spark-lib project. It only considers a .ply file a point cloud if it contains all of the following properties: x, y, z, red, green, blue. It also supports an optional alpha property, similar to how Spark 0.1 handled it.

Since point clouds don't have scale or rotation, a fixed scale of 0.001 is chosen for all three axis. This is the same default/fallback value as in Spark 0.1, but unlike the old approach, this value can't be configured as the constant is on the Rust side of the project.

It's likely that there are still differences in behaviour between this implementation and the old one, especially given that the old one mixed splat and point cloud handling (e.g. it was theoretically possible to have point clouds with spherical harmonics). Testing was done by loading a coloured point cloud file exported from CloudCompare

[asundqui]

Love it, thank you for bringing this feature back @mrxz . @dmarcos we should merge this right?

[dmarcos]

Thanks @mrxz