WIP: HltEfficiencyChecker for @abrossag use case

This MR (not for merge at this stage) is to try and use the HltEfficiencyChecker for @abrossag's use case. We want to be able to use the pre-made options file (renamed and put in HltEfficiencyChecker/options/arnau_hl2_example.py) to derive the input files and decay-specific information in order to run HltEfficiencyChecker with HLT2. Thus this MR builds off !10 (closed).

Then we want to add a couple of new tuple tools to the control flow so that @abrossag can do some specialised analysis on the output.

This is a helpful test case for generally using HltEfficiencyChecker with HLT2 lines, and commits made here can be cherry-picked to !10 (closed).

TODO:

• Get a working decay descriptor
• Add TupleToolMCBackgroundInfo and TupleToolMCTruth to the control flow
• Figure out if number of candidates > number of events is a big issue
• Add a DecayTreeTuple of reconstructed kinematics?

cc @tlatham @mkenzie