Add Game Context Info (like in FEN)

[Bonifatius94]

Issue:

• the chess board only keeps track of the chess pieces
• lets of params need to be handed in for further context info
• make the chesslib more user-friendly by handling all those context info

Suggested Solution:

• introduce a new data structure based on the bitboard format,, but extended by some extra context info
• it is supposed to be contextually at least as powerful as FEN (add game round, drawing side, etc.)
• call it maybe something like GameSession (because that's what makes it different from just the plain bitboards)
• rework the API to support applying draws to the game session, etc.