[Feature Request] Allow +ASC for mixture models

[HS6986]

Dear IQ-TREE Developers,

The ascertainment bias correction parameter +ASC is currently not applicable to mixture models, and attempts to apply it result in the message Mixture model +ASC is not supported yet. Contact author if needed. However, I think that it would be valuable to support it.

It would be useful when applying mixture models, for example, to binary microsynteny data without invariant characters (e.g., Vasilikopoulos et al., 2024) and neomorphic (see Sereno, 2007) binary morphological characters (e.g., Juravel et al., 2023). I am sure there are other situations where this would be useful.

Although of course we would have to give this proposal a lower priority than the others, I would like to promote it.