performance_model isn't checked before using

[jerry871002]

To pass the mypy type check, I initialized performance_model as None (see the examples below).

https://github.com/jerry871002/bayesian-strategy-inference/blob/e71229871278286cf72590ea3bff5508886092fa/src/grid_world/agent.py#L29

https://github.com/jerry871002/bayesian-strategy-inference/blob/e71229871278286cf72590ea3bff5508886092fa/src/navigation_game/agent.py#L27

https://github.com/jerry871002/bayesian-strategy-inference/blob/e71229871278286cf72590ea3bff5508886092fa/src/soccer_game/agent.py#L29

But I didn't add any check before using it to ensure the value is properly set, e.g. is it properly set to an array-like value and not None. Need to fix this in the future.