[WIP][feat] Write a proper PLY conversion from and to a point cloud

Write a proper PLY Writer / Reader which can read PLY files for slam::PointCloud.

TODOs:

• Write a PLY Writer taking a slam::PointCloud as entry.
• Test the Writer
• Replace the previous io by the new writer (using the slam::PointCloud writer)
• Test that the writer and reader behave nicely on complex point clouds