Static topology of a robot — links, joints, sensors, and actuators.
Read via bb://robots/{robot}/topology. The contents do not change
after the robot is compiled.
Every value is in SI base units: metres, radians, and their per-second and
per-second-squared derivatives. A joint reports the fields its type gives it,
so axis, origin, limit and dynamics are each present only where the
robot declares them.
axis is the joint's axis of rotation or translation as a unit vector, and
is what a planar joint's in-plane numbers from
bb://robots/{robot}/joints are measured against — it is that plane's
normal.