Correct the tank cost calculation.

At least, make it more correct. I don't know if the tank cost itself should
be scaled by utilization, but it seems appropriate that the cost should
have that factored in already.
Make the default base cost 0.
Put utilization back into the tank cost.

I had a chance to talk with Nathan and it's intended that the cost is per
tank volume, not contents volume.
Build the tweakscale stuff at all times.
Change L to u.

Stock resource units are not liters. Closest real-world unit is the British
gallon :P.
Merge branch 'master' into chainsaw