examples/Cars: car_jump.urdf is no longer loadable.

[rpoyner-tri]

Since the car_sim_lcm program can no longer load URDF format world models, car_jump.urdf is not usable. I propose either translating it to SDF or deleting it and updating the gallery documentation appropriately.

[liangfok]

I was able to get the demo to work by re-enabling URDF-based world models. Here are the commands using a ninja style build:

// Starts the Drake Visualizer.
$ cd drake-build
$ ./install/bin/drake-visualizer &

//  Starts the Steering Command Driver.
$ cd drake-distro/examples/Cars
$ export PYTHONPATH=drake-build/drake/lcmgen/lcmtypes:$PYTHONPATH
$ ./steering_command_driver.py

// Starts the Drake Simulator.
$ cd drake-build/drake
$ ./bin/car_sim_lcm /Users/liang/dev/drake-distro/drake/examples/Cars/models/prius/prius.urdf /Users/liang/dev/drake-distro/drake/examples/Cars/models/car_jump.urdf

Here's a screenshot: