MuonReadoutGeometryR4: Fix cppcheck warnings.

Prefer returning const references to members rather than copies.

This merge request affects 1 package:

• MuonSpectrometer/MuonPhaseII/MuonDetDescr/MuonReadoutGeometryR4

This merge request affects 2 files:

• MuonSpectrometer/MuonPhaseII/MuonDetDescr/MuonReadoutGeometryR4/MuonReadoutGeometryR4/MuonReadoutElement.h
• MuonSpectrometer/MuonPhaseII/MuonDetDescr/MuonReadoutGeometryR4/MuonReadoutGeometryR4/MuonReadoutElement.icc

Adding @kluit ,@markowen ,@ddicroce ,@wleight ,@apsallid ,@jojungge ,@stavrop ,@mvozak ,@pscholer as watchers

added MuonSpectrometer Upgrade main review-pending-level-1 labels

CI Result SUCCESS (hash ccc8f953)

	Athena	AthSimulation
externals
cmake
make
tests

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Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
For experts only: Jenkins output (remote access info)

removed review-pending-level-1 label

added review-approved label

merged

mentioned in commit 62a9c670