Adding ReadDecorHandleKey for ptcone20 in the electron mon algos

Edmar Egidio Purcino De Souza requested to merge eegidiop/athena:fpefix into 23.0

This MR is adding ReadDecorHandleKey for iso variables for electrons, in order to only make the retrieving after this var be created. Discussions in ATR-26568 .

ping @safarzad , @tamartin , @christos , @wlampl and @fwinkl

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