Provide basis unit quaternions i j k

Unless I'm missing something in the documentation, it appears there isn't any equivalent to `Vector3::x_axis()`, etc for `UnitQuaternion`s. The only way to construct `i` `j` and `k` without costly normalizations seems to be to use `new_unchecked`, as in `UnitQuaternion::new_unchecked(Quaternion::new(0.0, 1.0, 0.0, 0.0))`. It would be convenient to have static functions like `UnitQuaternion::i()` for this purpose.