Add weight_on_wheels method

[natsirt721]

Summary

Bugfixes "Muscle-powered vehicles counting animal weight for traction purposes"

Purpose of change

Fixes #73341

Describe the solution

When calculating traction and rolling resistance, the vehicle code used the entire weight of the vehicle to determine the loading that each wheel had. This did not account for the fact that beasts of burden are supporting their own weight and not contributing to the loading on the wheels of the vehicle they are pulling. This caused the issue seen in the original report - the 250kg wagon's wheels were bearing the weight of 2200 kg of horse, causing extremely low offroad traction and acceleration.

This changes adds a new function weight_on_wheels which returns the mass of the vehicle minus the weight of any harnessed creatures. This function is used in the two places where the mass of the vehicle is used to calculate loading effects on the wheels - traction and rolling resistance.

Describe alternatives you've considered

Making this a cached value that is calculated whenever mass_cache is. I didn't notice any performance issues but I'll admit that I wasn't looking for them.

Testing

Spawned two wagons and upgraded one of them to a four-horse configuration. Examined the rolling resistance and offroad rating of each with and without harnessed animals and observed no meaningful difference. Did some racing both on and off road and with a load:

horse count	surface	load	time to 16 mph
2	road	0	10s
2	road	450 kg	14s
2	dirt	0	10s
2	dirt	450 kg	n/a*
4	road	0	13s
4	road	450 kg	19s
4	dirt	0	13s
4	dirt	450 kg	31s

• the vehicle topped out at 13 mph around 28 seconds

Without a load, both vehicles have an offroad rating of 100%, so they perform the same on dirt and pavement. With the load, this drops to 20%, and the power from the additional horses makes the difference

Additional context