Add `plyToTurn` util function

[kraktus]

It's defined in three different places in lila.

[niklasf]

Looks a bit strange to me. Generally halfmove/ply is 0 based. And same in lila, for example on the analysis board:

{"treeParts":[{"ply":0,"fen":"rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"}], ...}

But somehow in all these contexts lila uses 1-based ply? :thinking:

[kraktus]

It could be a historical mistake that has been copied over and over? I haven’t checked back but it seemed only used for user facing information

[niklasf]

Sorry, just noticed the PR is still hanging here. It seems to be working out fine in lila, but still a bit unusual method of counting.