Feature: parallel environment

[braintimeException]

Hi!

I have implemented a new feature. It is an environment which handles many instances of HanabiState running in parallel for one instance of HanabiGame.

The main benefits of this approach is that it

• avoids the overhead of having several games
• is easily parallelized on the level of individual states
• the vectorized observations, scores, etc. are efficiently exposed using numpy arrays.

As an example, I am developing a hanabi multi-agent framework based on this implementation.

I thought it might be useful for other people as well, so I decided to open this PR.

Cheers,
Anton

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[braintimeException]

@googlebot I signed it!

[googlebot]

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