Add a LinkNavigator utility

[tmadlener]

BEGINRELEASENOTES

• Add an LinkNavigator utility class that facilitates the lookup of associated objects

ENDRELEASENOTES

The main working principle is:

// From MC to reco particles
auto& mc2recoAssocs = event.get<MCRecoParticleAssociationCollection>("MCRecoTruthLink");
auto mc2reco = LinkNavigator{mc2recoAssocs};

auto& mcps = event.get<MCParticle>("MCParticles");
for (const auto p : mcps) {
  const auto recos = mc2reco.getAssociated(p);
}

recos in this case is a std::vector<WeightedObject<ReconstructedParticle>>, which effectively just couples an object and a relation weight together. The foreseen usage is something like this:

for (const auto p : mcps) {
  for (const auto& [reco, weight] : mc2reco.getAssociated(p)) {
    // do something with reco and the weight
  }
}

• Needs (includes) Allow for relations between two arbitrary types #255
• Documentation
• Handling of std::is_same_v<FromT, ToT>