Add possibility to use range based for loops on relations and vector members

[gaede]

Add a helper class vector_view and code generation such that on can write range based for loops, e.g.

auto mcps = store.get<MCParticleCollection>("MCParticles") ;
for( auto& p : mcps ){
    for( auto& d : p.getDaughters() ) {
        printDaughter(d) ;
    }
}

[tmadlener]

This can be closed, as it was addressed by #107 (Wasn't closed automatically, because at the time of merging, the issue was not yet linked to the PR).