IDC: Class trait tweak

std::pair wasn't doing what I expected so swapped it for a defined struct.

This merge request affects 1 package:

• Event/EventContainers

added EDM master review-pending-level-1 labels

CI Result SUCCESS (hash 8c0a7c51)

	Athena	AthSimulation	AthGeneration	AnalysisBase
externals
cmake
make
required tests
optional tests

Full details available on this CI monitor view
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 18853]

added review-approved label

removed review-pending-level-1 label

merged

mentioned in commit ecc8f631

added sweep:ignore label