Document Si track extension - first batch

This MR contains the initial work towards cleaning up and documenting the combinatorial silicon track extension - including the SiTrackMaker_xk, SiCombinatorialTrackFinder_xk, SiTrajectory_xk, SiTrajectoryElement_xk.

The changes attempt to make the code more readable for newcomers and document the logic in a more accessible way to users studying the code, while preserving the elegance of the original implementation. No performance changes are expected.

As the classes are tightly coupled, they can only be documented together. As a result, there is a large number of changed lines throughout the affected classes, even though the vast majority of changes are just formatting, renaming, and comments.

Given that I have reached 1400 lines of changes, with a related growth in the risk of merge conflicts (already had one), I am containing the progress so far within this MR, and will follow up in another.

Adding @sroe, @npetters, @sswatman and @zschilla

Also adding @jsandesa - do any of my changes clash with developments you are working on?