This MR aims to remove the use of plain decorators in METEgammaAssociator, as flagged here: https://its.cern.ch/jira/browse/ATLASRECTS-6100 The plain decorators are replaced by DecorWriteHandles. Please note that I have not touched the accessors in the code (i.e. const static SG::AuxElement::ConstAccessor PVMatchedAcc("matchedToPV"); ) but am happy to do this if it is required. Tagging @wbalunas and @sawyer from JSV, and @mhodgkin and @goetz from the reconstruction domain