Reduce memory and persistence footprint of SingleObjectiveTestFunctionSolution

[swagner]

In the SingleObjectiveTestFunctionSolution the whole population is referenced in order to be able to visualize all solutions graphically. However, graphical visualization is only available in the 2D case. Therefore, the whole population should also only be stored, if the problem dimension is equal to 2. This would drastically reduce the memory and persistence footprint required when storing test functions optimization runs with high problem dimensions and population sizes.