The twist motion performs an angular rotation about the given axis _and_
the origin of ik_frame as well as a linear translation.
Both transforms are expressed w.r.t. the model frame and thus require
left-multiplication to ik_frame's current pose.

The ik_frame should move in a straight-line Cartesian path.
However, so far the link frame was following a Cartesian path.

When passing the root frame, getRigidlyConnectedParentLinkModel() returns
a nullptr for robot_link, causing a segfault.
Actually, we don't need to use that method at all. We just need to find
the robot_link of an associated body.

- link_pose -> start_pose
- pos_link -> pos_start