links: 
base
link_0
link_1
link_0_helper

joints: 
joint_name: joint_0 joint_type: continuous parent_link: link_0_helper child_link: link_0
joint_name: joint_1 joint_type: fixed parent_link: base child_link: link_1
joint_name: joint_2 joint_type: prismatic parent_link: link_1 child_link: link_0_helper
