Make the offer process learned (rather than rules-based)

[thobson88]

Currently offers are determined by fixed rules based on the agent's inventory.

For truly emergent trading, we need decisions on offers to be part of the action space.

[edchapman88]

Suggested process:

• Trader uses the wrapped Forager's posted_offer field to resolve possible trades (this process must ensure offers are still valid at this point)
• As before, Trader's are ensured to only trade once using the traded vec
• automatic resource consumption takes place
• Forager takes an action based on agent_state (which now should include posted_offer)
• check posted_offer is valid, given the inventory levels of the agent (if not, set to trivial - agent must learn which offers are possible given resource levels)

[edchapman88]

Useful analysis:

• episode vs. automatic posted_offer cancellation (due to incorrectly judged offer action)
• episode vs. average time with offer posted
• episode vs. rate of successful trades