Kalman tracker: a tracker the deals specifically with linear motion.

Nothing fancy: we just use a Kalman filter to build predictions, that we link to actual spot thanks to the LAP tracker. 

It again deviates from the Jaqaman paper by being somewhat simpler and probably less accurate. The PR is here because the tracker is functional, but most likely it will be changed as we get applications for it, and some concrete measure of its performance to optimize it.