RFC: Simple fuel price system

[MasterOktagon]

Right now we do have running costs that are dependent on the vehicle type. This is fine for most cases but would like to have an option to enable a simple fuel price simulation to favor using steam locomotives in the early game, diesel in the mid game and electric in the late game. This would make vehicle buying decisions decisions a lot more varied over the course of a game (I mostly end up upgrading directly from steam to electric) and maybe let you have some different locomotives in you depots to swap in certain situations.

I imagine the system to work as follows: There are three running cost multipliers, for steam, diesel and electric respectively.
These would change over the years according to some curve, in the best case this curve should be modifiable via NewGRFs
example curve (btw how do I scale an image?):


With inflation enabled the inflation should also increase the multipliers accordingly.
With recessions enabled, there should be events such as "fuel shortage/coal shortage/energy crisis" (increased multiplier for one of the motor types for a certain time span)
or "fuel/coal/power subsidies" (decreased multiplier for a certain time span).

This would also need some minor UI adjustments such as adding a fuel price graph and changing the running cost to show something like "3.000t of coal/month" instead of "£3000/month"

obviously you should be able to toggle this behavior and change the recession impact in the settings.

I am probably able to implement it with a little help about the codebase.

[BW89]

I like the Idea, but i would recommend that electricity should always be cheaper than diesel and coal. The cost factor that makes it more expensive in some cases - the infrastructure (overhead wires, substations, ...) - is already handled by the infrastructure maintenance costs which are higher for electrified rail. This would cause a realistic incentive for using diesel on low use lines and electricity on lines with higher usage.