Improve documentation of the SiSpSeededTrackFinder and SiSpacePointsSeedMaker_ATLxk

This MR, as part of https://its.cern.ch/jira/browse/ATLIDTRKCP-263, attempts to improve a bit the readability of the track finder top level algorithm, as well as the seed finder tool.

In particular, comments were added explaining what is being done, and variable names were changed to be hopefully a bit more expressive.

In addition, a few C-style arrays were replaced by std containers in non-performance-critical locations.

This MR should not affect the reconstruction outcome or speed. This was tested on 100 mu=60 ttbars, with no change in the number of seeds/candidates/tracks and a track finder algorithm CPU stable within the resolution my local desktop allows (~1-2%)

Adding @sroe, @goetz for IDSW, and @npetters for IDTR