link_state velocity, acceleration, and wrench data should not use pose type

[scpeters]

Desired behavior

The following elements in the link_state specification all use the pose data type, even though the rotational components should not be represented by a quaternion.

• //state/link/velocity
• //state/link/acceleration
• //state/link/wrench

I'm guessing this is because there is no vector6 type to handle the spatial vectors. Since it can be confusing to remember whether the translational or rotational components should go first (SDFormat and URDF pose put translation first, while DART and many spatial algebra formulations put rotation first), an alternative is to split them into two vector3 types.

Alternatives considered

We could add a gz::math::Vector6 to support a vector6 type, but I think it may be less confusing to use two vector3s.

Implementation suggestion

Additional context