The PR summarizes the changes implemented during the ACTS workshop in Berkeley. It adds a new algorithm, a Sequential Kalman Filter to ACTS in its most simple form (just using the hit with the smallest chi^2 increment).
For this, we (@pagessin@nstyles Aaron) have implemented several things (and I took the liberty to add a few more stuff to this PR):
calculate the chi^2 in the GainMatrixUpdator
store it in the track state
add the algorithm itself
added a test for it
I have also refactored the Kalman Test because now both tests use the same basic procedures
