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?

InnerDetector/InDetRecTools/SiTrackMakerTool_xk/src/SiTrackMaker_xk.cxx

@@ -1150,16 +1150,22 @@ bool InDet::SiTrackMaker_xk::newSeed(SiTrackMakerEventData_xk& data, const std::
     /// incremenent counter again
     ++n;
   }
-  /// check if at least on cluster is not already used by a track. 
+  /// check if at least on cluster is not already used by any track. 
   /// This works since the multiset allows adding the same track multiple times
+  /// If this is the case, we accept the seed. 
   if(trackseed.size() < n) return true;
   /// in the case of HI reco, we accept any 3-cluster (PPP) seed. 
   if( m_heavyion && n==3 ) return true;
     
-  /// now we look for the track consuming the largest number of clusters 
-  const Trk::Track* currentTrack = (*trackseed.begin());
+  /// Now we look for the track consuming the largest number of clusters 
+
+  /// This is done by looping over all tracks using any of our clusters,
+  /// and counting the appearance of each track in the multiset. 
+  /// If one single track contains all of the clusters (--> is included n times), 
+  /// we reject this seed. 
+  const Trk::Track* currentTrack {nullptr};
   size_t clustersOnCurrent  = 1;
-  /// loop over the list 
+  /// loop over the list of tracks
    for(const Trk::Track* track : trackseed) {
     /// if this is a new track, reset the counter
     if(track != currentTrack) {