V1.2.0

It was decided to make a big new release of PyGMO when the following major overhauls will be achieved:

• Port to Python 3 [done][to be tested]
• Increase c++ Unit tests and restore some Python tests
• Clean population class and move multi-objective stuff to the utils namespace (change the current behavior of keeping the domination stuff updated in steady-state)
• Split chromosome representation in std::vector<double> and std::vector<int>
• Abandon cache in problem in favor of a set_x_if method set_x_if(x,lambda : prob.compare_fitness(prob.objfun(x),f2) ) (TO BE DECIDED AND DESIGNED WELL)
• Implement a polymorphic stopping condition class in the algorithm
• Check MPI functionalities ( its a long time since the code is untested on real cases )