[TME] Add explicit speed degradation

[Nomagno]

Description

Currently, there is an internal hardcoded rolling resistance factor. This was an interface design oversight, it should be exposed in kart_characteristics.xml too. DONE
There should also be a way to explicitly change degradation depending on speed.
The proposed interface is:

• In kart_characteristics.xml, add parameters usage_multiplier_turning, usage_multiplier_traction, and reference_speed_mult
• If the current speed is less than reference_speed_mult*kart_base_speed, nothing is done.
• If the current speed is more than reference_speed_mult*kart_base_speed, then:
  • usage_turning = normal_usage_turning * usage_multiplier_turning * (current_speed - (reference_speed_mult * base_speed))
  • usage_traction = normal_usage_traction * usage_multiplier_traction * (current_speed - (reference_speed_mult * base_speed))
• This effectively is a way to define a maximum speed, and then declare that for every kph above that maximum speed, the degradation will increase by a certain percent. The alternative is to instead do current_speed/(reference_speed*base_speed), which could be interesting too but has a different behaviour

[Nomagno]

Changes to the formula to make it match the intent (there were some errors), but otherwise implemented to spec.