Method synced_stream

include/BS_thread_pool.hpp:2415–2418

* @brief Construct a new synced stream which prints to `std::cout`. */

     * @brief Construct a new synced stream which prints to `std::cout`.
     */
    explicit synced_stream()
    {
        add_stream(std::cout);
    }

    /**
     * @brief Construct a new synced stream which prints to the given output stream(s).