MET (internal) overlap removal simplification

This commit introduces some simplifications to the internal overlap removal procedure of the MET calculation during systematic variations. In essence, the MissingETContainer terms re-arrangement is happening using nominally-calibrated jets even during a systematic variation pass. This leads to the same overlap-removal decision both at the nominal and any systematic variation pass. This is achieved by an additional decoration the METMaker::rebuildJetMET will look for called nominalObjectLink. This decoration holds an ElementLink<xAOD::IParticleContainer> to the nominally-calibrated jets, which should be created beforehand by the user at analysis-level. If this decoration is not found the algorithm falls back to the default behaviour.

NB: those changes are intended for short-term testing purposes by analyses. This is not an efficient implementation for the future as the overlap removal procedure is actually repeated in every systematic pass but gives the same result with the nominal pass.