Follow-up from "Algorithm that selects the particles used in the FlavourTagging"

The following discussion from Phys!1002 (merged) should be addressed:

• @graven started a discussion:

  The fact that this operator() returns a LHCb::Particle::Selection implies that it looses usefull information that it actually computed, namely which particles below to which bCandidate. So instead of returning a Selection it should return a Relation between the input B candidates and their corresponding flavour tagging particles.