Add mode arg to driver launch file

stretch_core/launch/stretch_driver.launch

@@ -2,6 +2,7 @@
   
   <param name="robot_description" textfile="$(find stretch_description)/urdf/stretch.urdf" />
   <arg name="calibrated_controller_yaml_file" value="$(find stretch_core)/config/controller_calibration_head.yaml"/>
+  <arg name="mode" default="position" doc="the initial mode for the driver" />
 
   <node
       name="joint_state_publisher"
@@ -34,6 +35,7 @@
   </node>
 
   <node name="stretch_driver" pkg="stretch_core" type="stretch_driver" output="screen">
+    <param name="mode" type="string" value="$(arg mode)"/>
     <param name="rate" type="double" value="30.0"/>
     <param name="timeout" type="double" value="0.5"/>
     <remap from="cmd_vel" to="/stretch/cmd_vel" />