[PWGLF] Add task for on-the-fly production of centrality strangeness derived table

[romainschotter]

• Add extra info in the event selection table for Run 2 (without introducing a new table version, but okay no one else uses the strangeness derived converted data for the moment)
• Fill event selection table for Run 2 with extra info
• Add task stracents.cxx to re-produce on-the-fly the strangeness derived centrality table, avoiding to regenerate the derived data in case a new centrality calibration is available. One can now just reproduce the centrality table with the new calibration