Method increase_value

tests/test_methods_and_attributes.cpp:154–157

public:
    void do_nothing() const {}
    void increase_value() {
        rw_value++;
        ro_value += 0.25;
    }
    void set_int(int v) { rw_value = v; }
    int get_int() const { return rw_value; }
    double get_double() const { return ro_value; }