FEN, PGN and arena adjustments for Chess960

[magehrke]

Small fixes for chess960:

• Adding fens for arena games in RL
• Adding correctly pgn printing for castling in chess960
• In arena mode, each opening is now played twice, where player are switched after each game.

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@QueensGambit Please have a look at the last commit. You said you wanted some changes regarding the usage of pointers?! Please let me know what I can do.

[QueensGambit]

Thank you for the update.
I changed fen to be a const string& datatype.
I think this makes the code easier to read and avoids an additional return parameter.