Adding a SwapColors option to explicitly swap colors in input encoding e.g. for knight odds

[Naphthalin]

Main application case for now is that when using a net trained from asymmetric training games (e.g. knight odds), this asymmetry will make use of the color to move input plane. Switching sides (so, using the knight odds net with black being a knight down) isn't easily possible since the NN still thinks white is the stronger player. This PR adds an option for pretending to be playing as the opposite color when sending positions to the NN, which might also be interesting for studying the learned white advantage bias in Lc0 nets, but most importantly for playing knight odds and similar as black.