Simplify cargo pods

[JensAyton]

Cargo pods have an internal cargoType property, which can be RANDOM, SLAVES, ALLOY, MINERALS, THARGOID or SCRIPTED_ITEM. RANDOM actually contains a fixed item determined by commodityType and commodityAmount. The other values, except SCRIPTED_ITEM, each represent one unit of a fixed commodity.

This reflects the cargo information stored in shipdata.plist for ships, but representing it this way for cargo pods seems entirely pointless. Pods should either be scripted or contain some quantity of a commodity. This information should be exposed to scripts. Scripted pods should also be able to set nominal contents, either as a string or a commodity with the extra option of none/null. (A script that wishes to display pod contents should represent null as “Unknown” or similar.)