PGA-5: Add stopping criteria to the GeneticAlgorithm

[gnypit]

In order to limit computational cost of executing the genetic algorithm, stopping criteria might be implemented:

• max number of generations to compute if the fitness value of the best solution doesn;t change, e.g. if for 50 generations the best solution remains the same, the algorithm is terminated;
• target value of the fitness, e.g. if fot the endurance of a material test case fit value 2.7 is high enough, once the algorithm reaches it, it is terminated.

Stopping criteria should be provided and stored as a parameter/field in the GeneticAlgorithm class, preferably passed to it in the constructor.