Adding a few features and fix

[BDonnot]

• [BREAKING]: allow the opponent to chose the duration of its attack. This breaks the previous "Opponent.attack(...)"
  signature by adding an object in the return value. All code provided with grid2op are compatible with this
  new change. (for previously coded opponent, the only thing you have to do to make it compliant with
  the new interface is, in the opponent.attack(...) function return whatever_you_returned_before, None instead
  of simply whatever_you_returned_before
• [FIXED]: an issue with the IncreasingFlatReward reward types
• [ADDED]: a reward EpisodeDurationReward that is always 0 unless at the end of an episode where it returns a float
  proportional to the number of step made from the beginning of the environment.
• [ADDED]: in the Observation the possibility to retrieve the current number of steps

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

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