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:

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