Add launch files for running demos.

[wohe]

This adds two launch files for running 2D and 3D demos.
A bag filename is expected as an argument and is played
back using simulation time while showing the result in a
preconfigured RViz window.

[damonkohler]

LGTM

[carl1218]

hi，
An error occurred while I was run the demo_2d.launch,
error messages：requires the 'bag_filename' arg to be set.
How should I set the bag_filename?
deeply sorry for this shallow question.

[rohbotics]

You can set it like this when you launch,
roslaunch cartographer_ros demo_2d.launch bag_filename:=<PATH TO BAG FILE>

Here is a link for reference http://wiki.ros.org/roslaunch/Commandline%20Tools#Passing_in_args