Implement the NSGA-III algorithm

[pmslavin]

This merge adds the NSGA-III algorithm to PagMo2, as described in Deb & Jain, 2014 doi: 10.1109/TEVC.2013.2281535.

The algorithm's behavior has been verified with a range of problems from the DTLZ and ZDT test suites along with a real-world water resource allocation problem. Unit tests are provided.

Note that this is in Draft status at the moment:

• Further docstrings are required.
• Class nsga3 methods should be accompanied by comments describing the section of Deb & Jain, 2014 they relate to.

[jschueller]

sounds cool, maybe rebase to current master so it could pass the tests

[pmslavin]

Thanks Julien, I'll do that. There are also some docstrings missing and a few inconsistencies in the nsga3 constructor arguments with respect to other algorithms, so I'll correct those at the same time.