Implement multi-contempt mode

[skiminki]

Implement multi-contempt analysis mode. The way this would work is as follows:

For every position:

1. Do Lichess masters db query
2. Clear tree when (a) more than one contempt level is specified; OR (b) both side analysis is used at non-zero contempt
3. Run Lc0 analysis at first specified contempt level
4. Run Lc0 analysis at second specified contempt level
5. ...
6. Produce: (a) PGN combining the contempt levels (if requested); AND (b) one PGN per contempt level (if requested)

This should speed up the analysis, since the NN evaluation cache is shared between Lc0 analyses. The NN evaluation is where the majority of Leela search time goes, even with a strong GPU such as RTX 4090.

Contempt level would be specified as something such as:
--analysis-pass=contempt,search-nodes:[double-check-nodes]

Further, we should get rid of separate white/black engines. Instead, one engine is always used, but the user could specify analysis sides white/black/both.

A warning should be produced if the user has not specified NN cache, or has specified a small NN cache than the maximum nodes per move at any contempt level.