Dual Quaternions for Rigid Body Transforms

[Dual quaternions](https://en.wikipedia.org/wiki/Dual_quaternion) can represent all of rigid body transformations (translation + rotation), with some advantages over matrixes:

- Better numerical stability
- Smaller memory footprint (8 values, two algebraic constraints)
- Small processing speed improvement (one paper I found said ~10% over matrices)

They also keep the simplicity of combining transformations is just multiplication.

A downside is that they only work for 3 dimensions (if I understand correctly), but given a large portion of users likely will be using only 3D geometry (for graphics, etc), it might be worth it. Also, there is already support for non-dual quaternions, so there is some interest.

Many of you probably already know this, just thought I'd document the case for them as I understand it.

If people are interested in this, I'll create a fork and add the feature? This is my first time contributing to an open-source project on github, so I may need a little guidance... Or, if someone else is interested in writing it that would work as well.