bug fix for PSO GEN

[Sceki]

This PR aims to fix a bug in the pso_gen algorithm.
Instead of

                        for (decltype(x1.size()) k = 0u; k < x1.size(); ++k) {
                            if (ub[j] > lb[j]) {
                                acc += (x1[k] - x2[k]) * (x1[k] - x2[k]) / (ub[k] - lb[k]) / (ub[k] - lb[k]);
                            } // else 0
                        }

it should be :

                        for (decltype(x1.size()) k = 0u; k < x1.size(); ++k) {
                            if (ub[k] > lb[k]) {
                                acc += (x1[k] - x2[k]) * (x1[k] - x2[k]) / (ub[k] - lb[k]) / (ub[k] - lb[k]);
                            } // else 0
                        }

As j spans through the pop_size, whereas k spans through the dvs size (which is the length of ub, lb).. This error was not spotted in the current tests because the pop_size were always chosen to be smaller than the dvs size.. thus shadowing the error.

[darioizzo]

A bug indeed. Nice catch. We tried with @bluescarni to understand when and how it was introduced, its likely it was me that when coding pso_gen decided to check the bounds in a separate for loop and mistyped the indexes. The issue was not caught as pso_gen has been rarely used since without bfe pso is almost always a better option. The reason pso_gen was coded at all in pagmo2 is to allow neuroevolution, since we used this algorithm during the ACT-MIT SPHERES experiments.

Its a definite merge for me.