Integration of ros_maps_to_pedsim for ROS map conversion

[fverdoja]

This PR addresses #38 and follows from the discussion there.

• added the package ros_maps_to_pedsim as submodule in 3rdparty,
• added a reference to the package in the README.md
• added an extra ROS param to sim_visualizer to change wall_marker_scale. The default of the wall_marker_scale param is left to 1.0m for compatibility with the rest of the codebase.

[ksatyaki]

This is cool, over in my fork, I have a simple executable to convert pedsim scenario files into ros png/yaml files.
This is even better since writing the scenario files are harder than creating maps! :-) 👍