VEH-53: Recording of Mileage/Economy against Fuellog entries

Need to decide how Economy/Mileage should be attributed as there's a deviation between charts at the moment (see VEH-52).

Logically, it should be as follows

|Litres|Odometer|Miles gained|MPG|
|10|10|10|1|
|10|20|-|-|

As we haven't had a fill since the Odometer reading of 20, we don't know what the mileage/MPG from that fill is.

The way we're displaying in the fuellog is

|Litres|Odometer|Miles gained|MPG|
|10|10|-|-|
|10|20|10|1|

The Stats run, however, correctly attributes the miles (in this case 10) back to the previous fill.

It probably makes more sense to do it this way, though it will require some adjustments to the Fuellog generation module, the stats module etc.

Better to do it sooner than later, especially as will need to clear the stored Eco data from the Live API if the changes are made