streamline tagging functors implementation

Replace inheritance of common code in TaggingDecision, TaggingMVAOutput and TaggingMistag by composition: instead of those three functors inheriting from TaggerFn, add a dedicated functor, FLAVOURTAG to obtain a given LHCb::Tagger, and replace the three functors with GenericFunctors which, given an LHCb::Tagger will return the relevant observable.

In addition to simplifying the code, and making it fit better in the rest of the functor design, the additional functor can, with DaVinci!1376 (merged), be used to directly to fill an LHCb::FlavourTag in FunTuple. Hence DaVinci!1376 (merged) requires this MR, as it explicitly uses the new FLAVOURTAG functor.