Kalman filter prototype

This PR is an early preview of Kalman filtering to 6D tag pose. Comes with hardcore c++11 `std::map::emplace()` and related syntax :)

Is a partial solution to #19.

For now, the quick & dirty covariance calibration seems to work decent enough. Please test further.

~~A huge issue now is gimbal lock and discontinuities in angle. We need quaternions to fix this.~~ done

In the near future, more goodies such as 

- ~~Calibration API~~ done
- ~~Enable/disable on demand API~~ done
- ~~Quaternions~~ done
- ~~IMU integration~~ done, see also https://github.com/chili-epfl/qml-imu
- ~~Optimizations~~ pretty much done
- ~~Sample~~ done
- ~~Remove `Filter` completely~~ done
- ~~Add tag discard mechanism~~ done
- ~~Fully document all math involved~~ done

will follow. 
> the only interaction with this now is qml-imu
you're not exactly giving other users a chance of being represented here ;)

> At no point would the user ever enter values by hand there.
How about a scenario where users would film a rotating object for example (e.g. a 3D scanner) ? Or tests actually ;) Where you would have inputs simulating camera movement.

Even if the user does not set values by hand, the question is whether we're 100% sure that all these IMU/odometries will have quaternions as output. If not it can be quite confusing to have a mixed quaternion/4x4 interface... Is the conversion of one matrix per frame so expensive ?