Created a PTX file reader.

This is a PTX file reader. It has the quasi-obvious limitation that it needs to scan the full file to have the bounding box.
Used together with the tiling patch it allows to index a pointcloud directly from PTX files, without the need for txt2las or lastile.
By now it is not useful without the tiling phase, because each ptx file will be scanned many times only to have its bounding box during indicization.