Support SharedObjectsContainer in DecayTreeFitter

Change the input container type of the DecayTreeFitter algorithm from LHCb::Particles to Gaudi::NamedRange<std::vector<const LHCb::Particle*>> in order to accept both KeyedContainer (like LHCb::Particles) and Gaudi::SharedObjectsContainer (returned by filter algorithms like FilterDecays).

To be merged with DaVinci!670 (merged).