- add code to support Tagger::TagResult as strong enum
- getters return enum instead of short int
- setters require enum instead of short int

- improve use of flags by using generic functions to set/get,
  and compute offsets from values, instead of defining them
  coherently
- add [[nodiscard]] where appropriate
- avoid comparing SmartRef to 0
- have setters return reference to *this
- declare operator<< into the smallest possible scope so it does
  not pollute the global namespace -- and implement in in the .cpp
  file instead of in the header file. Also, prefer '\n' over std::endl
  to avoid flusing the stream midway

this allows a Relation of Particle -> VertexBase to be created
from links to a container of RecVertex (which derives from VertexBase)
by explicitly recognizing that the target container is not a container of
VertexBase, and trying whether it is a container of RecVertex instead

and make `unpack` return a StatusCode to know when an unpacker has
failed, in which case, if available, another unpacker for the same
CLID can be tried

- in addition, for backwards-compatibility, add fall-back
  in case a SharedObjectsContainer is requested to be unpacked,
  but a KeyedContainer is packed