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First attempt at moving HLTNav slimming online

Tim Martin requested to merge tamartin/athena:p1_nav_slim into master

Perform navigation compactification at the end of running the trigger

  • The content of HLTNav_Summary (including the single terminus node), and the content of HLTNav_.* (which is O(1k) collections) are all copied into a single collection HLTNav_Summary_BSSlimmed
  • The terminus node is added first in this collection, such that it can be found easily again later.
  • We take this opportunity to take the re-mapped ElementLink data which is decorated on to HLTNav_.* and set these values as the default in the nodes in HLTNav_Summary_BSSlimmed. This saves us yet more space.
  • The payload of the navigation is unchanged. No nodes are removed, no ElementLinks are removed from any nodes. The indexing of the nodes in the collection obviously changed given that we go from O(1k) collections to 1 collection. But their interlinking via ElementLinks should remain unchanged by the compactification, hence the represented graph remains unchanged.
  • Because the slimming alg has to exceptionally run after the HLTEDMCreator (which is what is decorating the remapped feature's element links) we need to add a special-case such that this doesn't create a dummy HLTNav_Summary_BSSlimmed collection in line with the fixed EDM.
  • We change the offline first-preference ReadHandleKey to be HLTNav_Summary_BSSlimmed. This first-preference is used when we cannot infer the file's content (i.e. BS).

Opening for @cleonido

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