[CR] [WIP] Diesel engines. ( Rework of PR #5182 - Biodiesel engine )

Well guys, I'm back. Decided to resurrect this seeings how it seems to be a PR that some were anticipating. I tried to reuse my old branch, but in my absence the code has become incompatible ( at least for me to wedge my code in :D ) and it would be easier to start from scratch.

Here is what is done so far:

- Variable size V6 diesel engine
- Diesel fuel
- Diesel fuel tanks in three flavors: normal, small, and external.
- Ability to siphon both gasoline or diesel fuels.
    ( Both through the vehicle GUI and by activating the rubber hose item )


Here is what is planned:

- Biodiesel 
    ( This is the brick wall of this project... See below )
- Diesel pumps at gas stations 
    ( Will be rare... I'm thinking somewhere around 1 in 4 stations... Comments? )
- Biodiesel pumps at gas stations
    ( Very rare )

Now, here is where I need help... I can not for the life of me get the tanks to accept biodiesel. Also, even if I could get the tank to accept biodiesel, it would "magically" be turned into normal diesel if it were to be siphoned back out. Any ideas on what I could do about this? I really don't want to have a separate engine for biodiesel, although if possible separate tanks would be awesome :)