Quick update that hopefully completes the fuel usage simulations...

1. Added ability to purchase fuel at all ports.
2. Fuel now costs money to purchase (£1 per litre)
3. Fuel can also be sold at ports but at a loss.
4. Fuel levels are now stored against all owned vessels
5. Fuel levels are now saved in the save game file

Unless I find any major challenges this update should move into the live build on Friday.