DualQuaternion
UnitDualQuaternion
This implements `UnitDualQuaternion` as an alternative to `Isometry3` for representing 3D isometries, which also provides the `sclerp` operation which can be used to perform screw-linear interpolation between two unit dual quaternions.
This commit introduces the `DualQuaternion` type, in line with the plan laid out in [#487]. [#487]: https://github.com/dimforge/nalgebra/issues/487