write YAML for ROS map_server

[kingsimba]

I'm aware cartographer_pbstream_to_ros_map can convert pbstream to PGM and YAML which can be fed into ROS map_server

But the quality is obvious not as good as cartographer_assets_writer because it doesn't use all the laser scans(scan accumulation).

So, I modified ProbabilityGridPointsProcessor to generate the YAML file.

Related discussion also found on: cartographer-project/cartographer_ros#475

[bochen87]

Hi @kingsimba , thank you for your contribution. can you please sign your commits? The easiest way is to use git commit --amend --no-edit --signoff. See https://github.com/cartographer-project/cartographer/blob/master/CONTRIBUTING.md for details. Thanks!