WIP: Feature/sequential kalman filter

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

For merging this PR, I still need to

• add comments
• optimize includes

Comments are very welcome!