Add seed of the agent

[BDonnot]

Is your feature request related to a problem? Please describe.

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For now, everything can be seeded to have reproducible experiments. The environment, the chronics etc (some seeding are missing in VoltageHandler and Opponent).

But the "seed" feature is missing from the Agent interface. This can be problematic for some experiments.

Describe the solution you'd like

Add the seed() method into the Agent public API.

Modify the runner so that you can set the seed of the agent from there.

Rewrite notebook 4 so that it is correct (for now it's not correct at all).

[BDonnot]

Fixed in 0.9.3