- Handle includes a virtual method to define a grasp constraint
  - add a class AxialHandle that derives from Handle and that creates grasp
    constraints with free rotation around the x-axis of the handle.

  - when copying a joint, copy relative position in parent joint. Thus do
    not recompute it when adding child joint.

  return os at end of function.

  - TypeWkPtr -> TypeWkPtr_t.

  - build kinematic chain in Robot::init so that weak pointer to instance is
    already created,
  - attach joint to parent before attaching children.