The result of computing a joint. It provides AffineTransformations that
describe the orientations of the components of the joint.
All affine transformations reorient the +Z axis to aim in the direction
of the far point. For example, in
joint_arm_lower the +Z axis aims
at the position of the hand.
Compute a joint where given a bend vector, describing the direction in which the articulated point (elbow) will try to move when the arm is retracted, and shoulder or base of the joint, the total length of the joint, and ideal position of the hand.