Comment on how not to "optimize" the CSE.

From code review with Lee.
Introduce convenience function for vector magnitude.
Update noise figures for PSAS data.
Approximate quaternion update is too approximate.
Use Taylor-series expansion of quaternion update.

The Taylor series expansion of the quaternion axis-angle formula never
divides by any quantity that might be zero. It also avoids computing
fancy floating-point functions like sin, cos, or sqrt. And since the ARM
chip we're running on doesn't have those fancy functions in hardware,
this implementation is as efficient as we're going to get anyway.

Even fairly low-order approximations introduce error small enough that
it's swamped by other filter errors. I've chosen a third-order
approximation and found it had no more than ~10^-8 absolute error after
four thousand timesteps, compared to a full sin/cos-based
implementation.