Draft: Add anti-tau into MET computation (#130)

Comes on top of !854 (merged)

Note: all of the taus flagged as possible anti-taus are taken into account, without reduction of their number based on the event category. This is because the reduction of the number of anti-taus only happens when overlap removal is computed but the MET configuration explicitly discards any OR related selections, to perform internally its own overlap removal.

https://gitlab.cern.ch/atlas/athena/-/blob/main/PhysicsAnalysis/Algorithms/MetAnalysisAlgorithms/python/MetAnalysisConfig.py#L99

FYI @edson @pbokan @bmoser @mfujimot @shahzad