DTLZ improvements

[CoolRunning]

There is currently a switch statement in the DTLZL-problem that triggers certain objective functions to be executed on the decision vector. A potential performance improvement would be to substitute this call with function pointers or templates.

    switch (m_prob_id) {
        case 1:
            retval = f1_objfun_impl(x);
            break;
        case 2:
        case 3:
            retval = f23_objfun_impl(x);
            break;
        case 4:
            retval = f4_objfun_impl(x);
            break;
        case 5:
        case 6:
            retval = f56_objfun_impl(x);
            break;
        case 7:
            retval = f7_objfun_impl(x);
            break;
    }