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. 
It's not about looking nice, it's about being coherent. If you use chilitags to measure the move of one camera for example, this output is not usable as input.
And no, you're not the only user of chilitags.