Delegate random solution generation

[hdbeukel]

Refactor AbstractProblem into a concrete class by delegating random solution generation. Introduce an interface RandomSolutionGenerator for this purpose. This avoids the need to extend an abstract class for custom problem specification. Specific predefined problems (subset, permutation, ...) can still extend this class to provide a default, but customizable, random solution generator.

Question: appropriate new name for this class?

• GenericProblem
• UniversalProblem

[hdbeukel]

• Update SubsetProblem accordingly.
• Update UML, wiki and problem specification description.