Improve support to integers

[darioizzo]

At the current status integer programming support is limited. The integer part of the decision vector can be passed as an argument to algorithms and the evolve() takes care to apply different operators on the float and the integer part.

This way of doing things has obvious drawbacks and a better model would be to define an integer part of the decision vector in the problem.

• Introduce in the UDP interface a get_nix(), that similarly to get_nic(), get_nec(), defines the integer part of the decision vector. (check that get_nix() <= get_nx() and that the bounds corresponding to the integer components are indeed integers)
• Modify the population constructor so that random decision vectors will have integers in their integer part (maybe modify the pagmo::random_decision_vector utility)
• Update the nsga2 and sga algorithms removing the m_int_dim member and using the get_nix() from the pagmo::problem instead
• Update the algo list with the int information.

[darioizzo]

#115 Is adressing this

[darioizzo]

#115 is now merged into master ... minlp support will be available in 2.4