Change DetectorSpecs singleton accessor to use pointer instead of reference.

[hgreenlee]

Description

Change DetectorSpecs singleton accessor (DetectorSpecs::GetME()) to return a pointer instead of a reference. This avoids false positive dangling reference warning for base qualifier e28 / c++20 without pragmas or compiler options.

This is an interface change, but not a functional change. Code affected by the interface change is limited to sbncode package.

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• Does this PR affect CAF data format? If so, please assign a CAF maintainer as additional reviewer. @Gianluca Petrillo and @thomas Jones
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[hgreenlee]

Will use a different solution.