BUG: correct averaging bug in compute_system_rewards #4

Summary

Population.compute_system_rewards always returns the raw sum of per-agent stats instead of the average, inflating every value by a factor of N (number of agents).

Bug

The averaging loop has two problems — it modifies agent_reward (the local per-agent dict) instead of reward (the accumulated total), and it sits inside the for agent loop so the result is discarded on the next iteration anyway.

# buggy  [header-1](#header-1)
for agent in self.agents:  
    agent_reward = agent.compute_morphology_statistics()  
    for k, v in agent_reward.items():  
        reward[k] += v  
    # average — wrong variable, wrong scope  
    for k in agent_reward.keys():  
        agent_reward[k] = agent_reward[k] / len(self.agents)