Gather all outputs of LHCb::Particle producers
Until the entire selection and persistence framework respects
Gaudi::Functional principles, we cannot assume that only the
LHCb::Particle output of an algorithm will be used downstream.
For example a composite particle producer may also output some child
Particle objects or Vertex objects it created on the fly. These will be
referenced via pointers on the Particle object and so act as implicit
input whenever the Particle object is used.
To be safe, we assume all outputs of any Particle producer will be used
if the Particle itself has been used.
Closes #259 (closed).
This is required for !669 (merged) but can be tested independently.
/cc @cmarinbe (thanks for reporting!)