Expanded functionality of flags to better support tracking cuts setup

This MR is about supporting patter convenient for defining repetitively many flag containers that are then merged with the main set of flags in several places. The primary user is tracking, however, the existing use case of offline muon reco in trigger is also addressed in a more consistent manner.

In addition, I dropped py2 compatibility layer and added documentation. Taggin @iyeletsk @wlampl @emoyse @mhodgkin