Implemented parsing of keep statements and struct variables

[XiaoFei9704]

Description

Implemented parsing of keep statements and struct variables. Just like: # 设置速度 speed_a: velocity keep (default speed_a == 30kph) struct weather: fog: fog rain: precipitation struct precipitation: intensity: velocity struct fog: visual_range: length # 设置天气 w: weather keep(w.fog.visual_range == 150m) # 能见度 keep(w.rain.intensity == 10mps) # 降水量 Now the system can correctly recognize the meaning of this sentence. Next, we can call Carla's API to extend the Domain model.

Where has this been tested?

• Platform(s): Ubuntu 20.04
• Python version(s): Python 3.8.10
• Unreal Engine version(s): UE4
• CARLA version: 0.9.13

[XiaoFei9704]

Mainly modified in osc2_scenario.py file