Add Commodities

[matthew-parlette]

Commodities will be in-game items that can be:

• traded
• combined to build other commodities

Features:

• not all commodities will be available at all ports
• cost of an item should be based on a cluster-wide availability of that item

[matthew-parlette]

To complete this for now, there will be four commodities:

• passengers
• fuel
• organics
• equipment