Simplify Rotation Axis

[r6]

Rather than passing both a rotation axis and an rotation angle, these two parameters can be combined into one. The magnitude of the rotation axis can specify the amount of rotation. This prevents trying to rotate about the 0 vector.

Also consider allowing different units of rotation. Using 1 to represent a full rotation is more intuitive than using 2*pi. Providing multiple functions may be the way to go to solve this.