MATH: Remove redundant Euler conversion.

In this patchset, I expand the functionality of the Quaternion class and comment it. I also fixed a camel case issue in Vector3D. Finally, I removed the redundant Euler angle conversion in the Quaternion class and instead use the single implementation from Rotation3D.
Yeah, that's a good suggestion. I'll include that in the roll-up changes I'm hoping to push today. Thanks!