Provide a HasAtan2 Double instance with the class definition.
Move model noise parameters into the test drivers.

This way each user of the generic IMU sensor fusion model can have
different sensor noise parameters.
Add test driver for Paul Riseborough's test inputs.

This doesn't yet get remotely similar answers to Paul's though.
Fix orientation update equation.

1) The division by 2 was leftover from an earlier approximation.
2) Quaternion multiply is not commutative and was in the wrong order.