stepper_details.cpp example doesn't build because placeholders are not in scope

[LegalizeAdulthood]

The following code fails to compile:

    // Symplectic harmonic oscillator example
    {
        double t( 0.0 ) , dt( 0.1 );
        //[ symplectic_stepper_detail_example
        pair< vector_type , vector_type > x;
        x.first[0] = 1.0; x.second[0] = 0.0;
        symplectic_rkn_sb3a_mclachlan< vector_type > rkn;
        rkn.do_step( make_pair( harm_osc_f1() , harm_osc_f2() ) , x , t , dt );
        //]

        //[ symplectic_stepper_detail_system_class_example
        harm_osc h;
        rkn.do_step( make_pair( detail::bind( &harm_osc::f1 , h , _1 , _2 ) , detail::bind( &harm_osc::f2 , h , _1 , _2 ) ) ,
                x , t , dt );
        //]
    }

Because std::placeholders::_1 and _2 are not in scope. The fix is to include <functional> and add using declarations for the placeholders:

    // Symplectic harmonic oscillator example
    {
        using std::placeholders::_1;
        using std::placeholders::_2;

        double t( 0.0 ) , dt( 0.1 );
        //[ symplectic_stepper_detail_example
        pair< vector_type , vector_type > x;
        x.first[0] = 1.0; x.second[0] = 0.0;
        symplectic_rkn_sb3a_mclachlan< vector_type > rkn;
        rkn.do_step( make_pair( harm_osc_f1() , harm_osc_f2() ) , x , t , dt );
        //]

        //[ symplectic_stepper_detail_system_class_example
        harm_osc h;
        rkn.do_step( make_pair( detail::bind( &harm_osc::f1 , h , _1 , _2 ) , detail::bind( &harm_osc::f2 , h , _1 , _2 ) ) ,
                x , t , dt );
        //]
    }

Pull request to follow...