Add quaternions

[adambrewster]

There's a quaternion type in examples. I propose that this become a part of Base.

1609 mentions them as part of a graphics package, but quaternions are a useful mathematical concept outside of graphics, and belong in Base. If complex numbers made the cut, then quaternions should too.

[stevengj]

Quaternions are nowhere near as widely used as complex numbers; the comparison is absurd.

Quaternions are used in an extremely narrow niche of technical computing, and make the most sense to me as an external add-on package rather than part of Base. The difficulty with adding a new numeric type to Base is that then the expectation arises of supporting that type throughout Base (from linear algebra to special functions ... certainly any entire function, such as exp or erf, could potentially be defined for quaternions); this is a huge amount of effort with limited payoff. Whereas in a Quaternion add-on package, expectations of the Julia core team are more modest.

[StefanKarpinski]

I agree that Quaternions make a good add-on package. Similar to DualNumbers.jl (although I've heard of quaternions before, so maybe a bit better known ;).

[ViralBShah]

We should certainly take it out of examples and into a package. We need an owner for it though.

[StefanKarpinski]

Could live under JuliaLang but it would be better if someone who needs it and uses it regularly is willing to take ownership.