Wrap the WFG toolkit in PaGMO

[krzysztof]

We are quite up to date with popular single objective benchmarks (CEC2009/13, upcoming bbob package), while the current multi-objective benchmarks are DTLZ and ZDT. Wrapping the problems in the WFG toolkit (in the style of the other benchmarks) would make a nice addition to it:

http://www.wfg.csse.uwa.edu.au/toolkit/