Improve MissingETAssociationHelper handling
This changes MET-related objects and interfaces to pass MissingETAssociationHelper arguments by reference instead of pointer. This avoids the possibility of passing a null/invalid pointer to the functions using them, which would always result errors and could only happen by accident or incorrect usage. This new scheme has exactly the same functionality but is safer.
From the user perspective, this changes the interfaces IMETMaker and IMETSystematicsTool: The MissingETAssociationHelper argument is now a reference rather than a pointer, and is no longer optional (omitting the optional argument always led to a runtime error anyway, which was suboptimal).
All code which used these interfaces in Athena has been updated accordingly (including common CP algorithms). Analysis framework developers will need to update their interface usage accordingly if using these two directly.