Move from SG::AuxElement to DecorHandle in bbyyAnalysis
Follows general recommendations https://gitlab.cern.ch/easyjet/easyjet/-/blob/main/CONTRIBUTING.md#L64
Will also fix KF_mbb
with systematics, which otherwise would have had the values of the last systematics for all variations, because of the use of a non syst-aware static SG::AuxElement::Decorator<float> KF_MBB("KF1_Mbb")
FYI @fcurcio
Edited by Thomas Strebler