Add a flag to save all pixel hits

This adds a flag, BTagging.savePixelHits which saves the full pixel hit collection to the AOD. It complements the existing "trackless" b-tagging flags. It is a more aggressive option intended for lower pt b-tagging studies, especially in the trigger. Might be of interest to @sargyrop, @tmadula, @cpollard, @lbeemste.

I also cleaned up some terrible looking lambda functions in the btagging flags, I couldn't stand looking at them any longer.