[bug] Pick & Place `franka_gazebo` example broken on Binary packages

[rickstaa]

As explained in #305 the Pick & Place example given in the documentation is not working when installing the franka_gazebo package from the binary packages. This is because the installation does not include the models and world folders. This is quite a trivial issue that can be easily solved by merging #305.

How to reproduce

1. Install the franka_ros binary package (i.e. sudo apt install ros-noetic-franka-ros).

2. Source the global ROS workspace (i.e. `. /opt/ros/noetic/setup.bash').

3. Launch the Pick & Place example using the following command:

   roslaunch franka_gazebo panda.launch x:=-0.5 \
   world:=$(rospack find franka_gazebo)/world/stone.sdf \
   controller:=cartesian_impedance_example_controller \
   rviz:=true

4. See that the table and pickup objects are not loaded correctly.

TODOs

• Merge #305.