add a simple stop sign example demo8.py; move the time step out as a parameter

demo/demo1.py

demo/demo3.py

demo/demo4.py

demo/demo5.py

demo/demo6.py

demo/demo7.py

demo/demo8.py

demo/example_controller3.py

demo/example_controller5.py

demo/example_controller7.py

demo/example_controller9.py

dryvr_plus_plus/example/example_agent/ball_agent.py

dryvr_plus_plus/example/example_agent/car_agent.py

dryvr_plus_plus/example/example_agent/sign_agent.py

dryvr_plus_plus/scene_verifier/agents/base_agent.py

dryvr_plus_plus/scene_verifier/analysis/simulator.py

dryvr_plus_plus/scene_verifier/analysis/verifier.py

dryvr_plus_plus/scene_verifier/dryvr/core/dryvrcore.py

dryvr_plus_plus/scene_verifier/scenario/scenario.py