Conditional target transformations

[JSzitas]

Allow something like

transform( BoxCox(target) ~ .ispositive, target ) ~ ...
within the soothsayer call. The transform would be conditional on features (akin to how rules() work), but with an added base case, maybe of the form TRUE ~ target (optionally also transformed).

This would make e.g. using automatic boxcox transform much nicer (and we could enable e.g. AutoBoxCox where selection would be done by something like guerrero).