message: ADD: `connected_to` optional arg to `panda_gazebo.xacro`
  This provides a similar interface to `panda_arm.xacro`
diff: <!-- || Paolo Robuffo Giordano, Alessandro de Luca || -->
  <!-- =============================================================== -->
  - <xacro:macro name="panda_arm" params="arm_id:='panda'">
  + <xacro:macro name="panda_arm" params="arm_id:='panda' connected_to:='' xyz:='0 0 0' rpy:='0 0 0'">
  + <xacro:unless value="${not connected_to}">
  + <joint name="${arm_id}_joint_${connected_to}" type="fixed">
  + <parent link="${connected_to}"/>
  + <child link="${arm_id}_link0"/>
  + <origin rpy="${rpy}" xyz="${xyz}"/>
  + </joint>
  + </xacro:unless>
  +
  <link name="${arm_id}_link0">
  <visual>
  <origin rpy="0 0 0" xyz="0 0 0"/>
  