Convert imperial tank volume to the metric system.

There are two different ways to specify the volume of a tank. In the
metric system, the tank volume is specified as the water capacity, while
in the imperial system the tank volume is specified as the air capacity
at the surface (1 ATM) when the tank is filled at its working pressure.

To avoid mistakes, the tank volume is now always returned as the metric
volume. For imperial tanks, the tank volume is converted to the metric
representation.