Use unique_ptr to manage one-to-one-relations in the Obj classes

[tmadlener]

BEGINRELEASENOTES

• Use a unique_ptr to manage the OneToOneRelation pointers in the Obj classes.

ENDRELEASENOTES

I am not entirely sure why they were not managed by smart pointers before. It's possible that this was not possible before we introduced the MaybeSharedPtrs.

[hegner]

@tmadlener - ROOT got hickups previously with them. But that's plenty of years ago. Thanks for the improvement!