Quaternion Math

Since the Numerics modules cover complex numbers and operations, and you mentioned here (#6) that ShapedArrays are within the scope of the project, would quaternion math and possibly other algebras fall under the scope of Swift Numerics as well?

Use cases include everywhere 3D rotation is used, from games and visualization to simulation and control systems. A well-tested and well-documented implementation would be a big boon for a new math library. Some quick googling shows this is not in the C++ STL or a standard part of NumPy, but it is a standard part of MATLAB and part of [Accelerate](https://developer.apple.com/documentation/accelerate/working_with_quaternions) (maybe better suited as part of an update/rewrite of Accelerate?).

Start by reading the scope discussion in issue #6 and the Swift Numerics context around complex numbers and ShapedArrays. Review Apple’s linked Accelerate quaternion documentation and compare the proposed use cases. Done would require an agreed scope for quaternion math or other algebras, followed by defined implementation and testing requirements; this issue names no files or tests.

Written by the indexing model from the issue text.