Stockfish: Change NNUE file extension

[nguyenpham]

The current file extension .bin makes confused since it says so little information, can’t help to recognise from other file purposes. In chess, that extension (.bin) is used already for Polyglot opening books too.

I suggest to change to specific ones such as “.nnue”, “.sfbin”, “.sfnnue”... to delete any confusion/wrong use in long terms.

[vondele]

@nodchip what would be your opinion? I wouldn't be opposed to .nnue but I have no strong opinion.

[nodchip]

I also don't have any strong opinions. Though .nnue sounds good to me.

[stockchess]

.nnue logical and distinctive extension.

[vondele]

done