WFG multi-objective benchmark problems

[CoolRunning]

The ZDT and DTLZ problems are the most frequently used benchmarks in the literature of MOEA, however they are outdated, partially too easy for modern algorithms, scale poorly (if at all) and have other inherent flaws. The WFG test problems provide a better benchmark as they have more diverse feature landscape and try to address the weaknesses of ZDT and DTLZ. An extensive review on this issue was done by:

Simon Huband, Philip Hingston, Luigi Barone and Lyndon While: "A Review of Multi-objective Test Problems and a Scalable Test Problem Toolkit"

Implementing the WFG benchmark as problems for pagmo is needed.

[bluescarni]

Done in #298.