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. 
> Please give me one real world use case (e.g github page) of 2D chilitags usage.

The best of all: https://github.com/chili-epfl/metroscope ;)

Let's discuss the idea of removing 2D here : #84

But for now, I think it wouldn't hurt too much to just unremove filtering for 2D only... If I'm not wrong, we don't need a calledBy3D if Chilitags3D sets its mChilitags.setFilter(0,0); (like it used to, because even simple filtering didn't want to be done at both 3d and 2d level)