Add data structures for wrench, twist

[osrf-migration]

Original report (archived issue) by Steve Peters (Bitbucket: Steven Peters, GitHub: scpeters).

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To support getting the force/torque wrench applied at a joint, it would be convenient to return a wrench as a unified data structure. It could also be helpful to report linear and angular velocity as twist.

In dart these are expressed by a Vector6d. An alternative is a struct with two named Vector3d values, which is more obvious to interpret but perhaps less efficient for screw algebra.

cc @mxgrey @azeey