[suggestion] Make burnt inventory size optionally match that of fuel with mod setting.

For certain mods (for me it was Industrial Revolution 3) that change burnt inventory size of locomotives, the new fuel inventory set by the mod's settings (1 by default) do not match the amount of fuel slots.

To make Single Train Unit mod work with those mods better, an additional setting can be added to alter amount of spent slots (like is already possible for fuel), and set to 0 by default not to interfere with vanilla Factorio.

Feel free to ask me for more details, and thanks for the great mod!

[muppet9010]

I'm on a break from modding due to change in job. So if I restart modding I will have a look at this, but it could well be a long time.

If you are a modder yourself and can do a PR I can look at that much easier.