Add is_finished to TBB Processing Pipelines

There is currently no way to reliably tell that a processing is finished (beside checking the available counters). With TBB, the only way is to call g.wait_for_all() which is a blocking call. Since we don't want to block the main thread, we can add a task to run the flow graph in the background.

void activate() {
  task_arena a;
  a.enqueue([background_task([this]() {
    // Construct graph (from preconstructed Body)
    graph g;
    function_node< int, int > f( g, 1, []( int i ) -> int {
      return spin_for(i);
    });

    //Activate source
    src.activate();
    
    g.wait_for_all();

    // Set captured is_finished
    this->is_finished = true;
  });
}

...

std::atomic_bool is_finished = false;