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Add BTaggingBuilderAlg and other b-tagging components

Merged Dan Guest requested to merge dguest/athena:btagging-builder into master

This adds a few algorithms to handle the plumbing around b-tagging:

  • BTaggingBuilderAlg: creates a b-tagging object for each jet and links it to that jet. Useful for lightweight b-tagging.
  • JetToBTagLinkerAlg: assuming that the link from the b-tag object to the jet exist, this creates the link back. Most analysis code follows this jet to b-tag link.
  • BTagToJetLinkerAlg: reversed version of the above. Since release 22 the b-tag object should always have a link to the jet, but in some older AODs the only link went from the jet to the b-tag.

None of these are used in current reconstruction code, so @duperrin need not worry that they will break everything. The main motivation to have them is to enable lighter b-tagging (i.e. using only DIPS) in the trigger. We should now have the tools to create a (minimal) b-tagging collection, associate tracks to it, and run a simple IP-based tagger. The existing C++ code was too monolithic to support this.

Edited by Dan Guest