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. 
So you really think that taking `(3,0)` and `(3,1)` elements of the transform matrix is more difficult than e.g averaging `(x,y)` of quad corners (or harder to understand)?

I could agree with the increased computational load of 3D transforms but that's another issue; do we have any system that couldn't possibly handle the load from going from 2D to 3D?

By the way I can't find the references to Chilitags in https://github.com/chili-epfl/metroscope?