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. 
My wish is to choose the option with most benefits:

1. Mat4x4 that is coherent
2. Vec3 + quat that plugs very nicely into IMU (and most possibly any other odometry estimator) that uses its own KF that dictates quat in its state vector; and that bypasses quat-rotmat conversion (and conversion back)

If someone else comes along and complains that that input would be better suited to their application as a mat4x4, I'll be happy to change it.