Add Gravity Effect to Trucks and Transports

Trains are currently the only entity that consider the impact of gravity on their transport efficiency, a fine compromise to increase immersion.

There's no effect on belts moving material upward hill, nor an energy saving moving material downhill either. This may all be a awash in a typical factory built on level ground, but along greater distances could make for more meaningful choices between transport modes, and general factory placement. The same applies for the other transports (including molten could reasonably be allowed to flow downhill), with fluid pipes perhaps having additional visual flair applied to vertical sections including a pump. I would suggest transports change their energy consumption based on the elevation change between start and end of a segment, ignoring the complications of tracking where along a belt material is currently. I would retain belt speed for simplicity and predictability.

Trucks are the main contender here for being affected by gravity as their trade off with trains for long distance hauling is wider. I would have trucks consume more fuel and travel *slightly* slower moving uphill, and less fuel and *slightly* faster moving downhill. Visually the trucks could bellow more black soot (diesel) to represent this increased fuel consumption more clearly.