Extending moarchiving to 3 and 4 objectives

[ttusar]

@nikohansen could you help me determine which of the methods of BiobjectiveNondominatedSortedList would need to be implemented by a similar class for 3 and 4 objectives? Here's a list of all of them:

• add
• add_list
• contributing_hypervolume (of an element already existing in the archive; if the element is not in the archive yet, it returns hypervolume_improvement)
• copy ?
• discarded (maybe for testing purposes)
• distance_to_hypervolume_area
• distance_to_pareto_front ("Pareto front" should be taken with a grain of salt)
• dominates
• dominators
• hypervolume
• hypervolume_improvement (0 if the element is already in the archive and > 0 otherwise)
• in_domain
• infos
• make_expensive_asserts (a flag used for testing purposes)
• remove

[ttusar]

Some additional clarifications after a discussion with @nikohansen:

• self.hypervolume_computation_float_type is telling you which kind of float type (float or fraction - to be used where the precision is cancelled) is used in some parts of the code. Done for performance reasons.
• Similarly holds for self.hypervolume_final_float_type
• Try to not introduce new dependencies (the code is independent of fractions.Fraction right now)