Add volumetric clouds

[Madecu]

-2D Clouds have been replaced for the volumetric cloud system from Unreal Engine. -Clouds are controlled by one parameter in the "BP_CarlaSunSky" blueprint: The "Cloudiness" variable inside the instance editable struct called "Weather." -Volumetric Clouds have been implemented to the "BP_Weather_Opt", that controls the weather at runtime. -Clouds color change dependant of the sun altitude and overall cloudiness, to get a better integration with the sky.

[AntonioMata1]

Volumetric clouds implemented in the weather system. They now blend with the sky better using a color curve that updates the tint depending on the altitude of the sun.