Modify the tournament selection method

[sergiomrebelo]

The current tournament selection chooses the two parents independently. This way, it always selects the best parent and, it can promote a greedy evolution of individuals. This issue is even more important since we are working with small populations (see issue #43).

This issue concerns the refracting of the selection method to create a method that returns the two best individuals of the tournament. It is an improvement on the system aligned with the user stories that comprise the use of the system with exploratory purposes.

[sergiomrebelo]

We have implemented a constraint evaluation strategy that employs Stochastic Ranking and a ranking tournament approach. Since we have refactored the tournament method, we modify the tournament method (see issue #46)