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)?

Yes. I learned about averages and basic cartesian geometry before highschool, but I learned about matrices after high school. Even now, it's not obvious that it's (3,0) and (3,1) and not `(0,3)` and `(1,3)`, or in which frame the transform matrix is or what not. If you look at TUIO, which is highly popular for tabletop, they only give position, scale and rotation for example.

> do we have any system that couldn't possibly handle the load from going from 2D to 3D?

Apparently you do, if you're afraid to hit the CPU limit because of one conversion to quaternion per frame.

Chilitags are called Craftag in metroscope, they're basically Chilitags 1.0

As a maintainer, I wish I only had the mental load of cooking the perfect API. It's however negligible compared to having to negotiate every non trivial change to a PR, having to beg for tests, doing them myself. I could spend this time being productive on the template matching branch I started for example. Heck, I would even like to fix issues that no one wants to fix.