Refactor autotrading

[dominik-zeglen]

Using a full list of commodities when searching for the best trading route seems to be cheesy and doesn't allow facilities with higher buy prices to get needed goods in reasonable time. Let's try to introduce some variety to the ai world:

• autotrading should work only for selected subset of goods. It can be further expanded by fleet improvements (via research or commander perks?) so more goods can be included in this list. Begin with list of three commodities and see what happens next
• introduce commodity traders to ai fleet planning. They can roam the sectors to trade only selected commodities, eg. basic goods trader moving only food and water between facilities