Updated get_keg_cap function

Increased the amount held by a standing tank to more accurately reflect the usage of the component tanks. That is, combining four together to get 900 volume means you would have somewhere in the ballpark of a quarter of the material left over - essentially another tank. This means the tank is actually somewhere around 1125, which rounds up to somewhere around 1200 due to surface area not scaling linearly with volume. A more precise value could probably be determined for this, but 1200 is somewhere in the right ballpark and is convenient because it simply and neatly doubles the already-extant wooden keg's volume.