Allow WFG parameters to be set.

- Added a new class called WFGProblem that contains common functionality
  shared by all the WFG problems. In particular, it contains methods to
  set the 'M' and 'l' parameters (number of problems, dimensionality).
  This class also makes sure that the WFG problems are reinitialized
  when one of these parameters are set. This is unfortunately a bit ugly
  as the sub-problems need to be reconstructed when one of these
  parameters change.
- Updated the WFG-problems themselves by adding the initialization-logic
  in an initialization method.
- Changed the vepso.xml file to include examples of using the WFG
  problem-set.