Alternate fixed wing estimator by Paul Riseborough

This pull request is not ready for merging into master yet, but ready for general hacking / contributions.

Some important notes:

- Check out https://github.com/LorenzMeier/InertialNav (master branch) to a directory NEXT TO Firmware (so src/Firmware and src/InertialNav
- Compile as usual
- The estimator is in InertialNav/code/estimator.cpp

Right now there is most likely an accel interfacing issue, which can be resolved by switching to sensor_combined for accel, gyro and mag values. We should pursue either this and/or fix the individual sensor topics.

Contributions / hacking very welcome.

@thomasgubler I will continue tonight with this. But if you could familiarise yourself it wouldn't hurt.