feature: Introduce `maybe_owning_ptr`

[ChristianTackeGSI]

See the doxygen comments for some details.

Will be needed in #1416, #1430.

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Checklist:

• [X] Followed the Contributing Guidelines

[dennisklein]

Can you post a pseudo code example on how you want to use this?

[ChristianTackeGSI]

Real code example of usage options: https://godbolt.org/z/5EG5qz55j

Or did you mean on how this will end up in FairRoot?

See both #1416 and #1430: Both have naive T* fFoo; unique_ptr<T> fFooOwned; way of handling the problamatic parts. I want to replace that with a maybe_owning_ptr<T> fFoo;.

[dennisklein]

Real code example of usage options: https://godbolt.org/z/5EG5qz55j

Or did you mean on how this will end up in FairRoot?

No, thx, the above was what I was looking for :)

[ChristianTackeGSI]

Can you post a pseudo code example on how you want to use this?

Should we have examples in the doxygen docs?

[dennisklein]

Should we have examples in the doxygen docs?

Would be a bonus exercise, not required from my point of view. If you want to add more docs, I'd prefer some unit test coverage - as "living" usage documentation.

[ChristianTackeGSI]

Yes, let's create a catch2 test for this, after your catch2 PR gets merged. :-)