cleaned up numerical code to avoid the use of "thresholds"
Instead of taylor series approximations, it was determined through
simulation that sin(theta)/theta remains stable as long as theta != 0,
and the (1-cos(theta))/theta^2 coefficient uses a trig identity
to avoid the subtraction and introduction of error in the numerator.

ProtectedAcos no longer needed, so deleted.

AxisAngle fixed to use Normalize3d function that already existed.

Test changed to detect errors in small angle calculations.