message: Restore memory of last_flow in TVIntegrator::GetFlow() + simplify volume update
diff: @@ -146,9 +146,10 @@ void TVIntegrator::AddFlow(Time now, VolumetricFlow flow) {
  initialized_ = true;
  } else if (Duration delta = now - last_flow_measurement_time_;
  delta >= VOLUME_INTEGRAL_INTERVAL) {
  - volume_ = volume_ + ml(delta.minutes() *
  - (last_flow_.ml_per_min() + flow.ml_per_min()) / 2);
  + volume_ =
  + volume_ + ml(delta.minutes() * (last_flow_ + flow).ml_per_min() / 2.0f);
  last_flow_measurement_time_ = now;
  + last_flow_ = flow;
  }
  }
  