Enhancements for FunTuple

Involves the following:

• Bug fix related to multiple candidates when investigating dst with high multiple candidate rate.
• @mengzhen pointed out (2nd point) here that if the input is ParticleFilter (and not ParticleCombiner or ParticleCombinerWithPVs) it triggers an error message. @sstahl notes here that this can be fixed through changing input LHCb::Particles to LHCb::Particle::Range.
• Code generalisation comment from Gerard on Mattermost here and here (@graven ).
• Suggestion of DEBUG messages suggested by Eduardo and general clean-uphere.
• Gerard's comment on making custom error message handling as suggested here.
• Also investigate templating the functor output in book_tuple function as suggested by PK here.

FYI: @erodrigu , @pkoppenb , @mferrill and @powen