Fix trigger boxes of generated signs

[bernatx]

Description

Fix the trigger boxes of actors. Now the bounding_box of an actor like a TrafficSign returns the merged bounding box of all trigger boxes related to that sign. So, for example, for traffic lights, will merge and return a bounding box that fits all the lanes affected by that traffic light.

Where has this been tested?

• Platform(s): Windows
• Python version(s): 3.7
• Unreal Engine version(s): 4.26

Possible Drawbacks

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This change is 

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