message: ADD: World Argument to Gazebo Launch file
diff: <arg name="paused" default="true" doc="Should the simulation directly be started?" />
  <arg name="arm_id" default="panda" doc="Name of the panda robot to spawn" />
  <arg name="use_gripper" default="true" doc="Should a franka hand be mounted on the flange?" />
  + <arg name="world" default="worlds/empty.world" doc="Filename to a SDF World for gazebo to use" />
  <arg name="x" default="0" />
  <arg name="y" default="0" />
  />
  <include file="$(find gazebo_ros)/launch/empty_world.launch">
  + <arg name="world_name" value="$(arg world)"/>
  <arg name="paused" value="$(arg paused)"/>
  <arg name="gui" value="$(eval not arg('headless'))"/>
  <arg name="use_sim_time" value="true"/>
  