feat(franka_gazebo): add physics engine launch argument

Author: rickstaa

franka_gazebo/launch/panda.launch

@@ -7,6 +7,7 @@
   <arg name="paused"      default="false" doc="Should the simulation directly be stopped at 0s?" />
   <arg name="world"       default="worlds/empty.world" doc="Filename to a SDF World for gazebo to use" />
   <arg name="rviz"        default="false" doc="Should RVIz be launched?" />
+  <arg name="physics"     default="dart"   doc="The physics engine used by gazebo"/> <!--Phyics engines: bullet|dart|ode|simbody-->
 
   <!-- Robot Customization -->
   <arg name="arm_id"      default="panda" doc="Name of the panda robot to spawn" />