Stores Individuals which have been created through aggregation of all the 
            Personas provided by the various 
            Backends.
          This is the main interface for client applications.
          Linking and unlinking of personas and individuals is performed entirely through the aggregator. Personas may be linked together to form 
            individuals; for example, the personas which form individual1 and individual2 may be linked together with 
              another_persona to give a new Individual:
            var personas = new HashSet<Persona> ();
  personas.add_all (individual1.personas);
  personas.add_all (individual2.personas);
  personas.add (another_persona);
  yield my_individual_aggregator.link_personas (personas);
          
          The individuals which contained those personas will be removed when 
            IndividualAggregator.link_personas is called. Any personas in 
            those individuals which were not included in the linking call may end up implicitly linked to the new individual, or may be aggregated 
            into other new individuals.
          For example, consider the situation where individual1 contains two personas, persona1A and persona1B
            ; individual2 contains one persona, persona2A; and another_persona comes from 
              individual3, which also contains persona3A and persona3B. Calling 
            IndividualAggregator.link_personas on persona1A, 
            persona1B, persona2A and another_persona will result in individual1 and 
              individual2 being removed. A new Individual will be created containing all the 
            personas passed to the linking function. It might also contain persona3A and persona3B; or they might be in one 
            or two other new individuals.
          An existing individual may be unlinked to form singleton individuals for each of its personas: 
  yield my_individual_aggregator.unlink_individual (my_individual);
          
          Note that to link two individuals together, their two sets of personas must be linked together. There is no API to directly link the 
            individuals themselves, as conceptually folks links Personas, not 
            Individuals.
          Folks does not support having more than one IndividualAggregator instantiated at the same time. Most clients should use 
            IndividualAggregator.dup to retrieve the IndividualAggregator 
            singleton.