ATR-22749 - B-phys di-electron chain

As in run 2, this chain just selects on di-electrons. Thus, the name has been changed from bBeeXM6000t to bBeeM6000 to avoid the seemingly endless confusion on whether or not this trigger also uses tracks, like bBmumux.

A more appropriate name would probably be mirroring bDimu, but bDie doesn't strike me as a particularly great trigger name. Additionally, the "t" was a tight distinction referring to improved selection in run 2 from M6000, this is not needed for the new algorithm.

I've only implemented one simple chain for now: HLT_2e5_lhvloose_bBeeM6000_L12EM3. The L1 seed is not currently correct, since L1_2EM3 obviously will not run online with any appreciable rate, but in MC/rerunLvl1 reprocessing tests this is equivalent to our unseeded trigger, and somewhat simpler to implement as a first test of the algorithms, for validation.

The trigger is fully defined in the electron slice, like Zee and Jpsiee chains, which seems to work ok. It uses the TrigMultiTrkComboHypo (and associated tool) just like in Run 2, and I've currently just copied the muon code into identical functions for electrons. I'd prefer to template the functions so that we don't e.g. fix a bug in muons but forget to fix the electron code, but I wasn't sure of the best way of getting around the fact that muon tracks are accessed with muon->InDetTrackParticle() whereas electrons need to use electron->trackParticle().

edit: the relevant functions are now templated and can handle both electrons and muons.

If there are suggestions for this, please let me know and I can update the algorithms!