Reduce model-loop: minimize LMER/MERF dependencies in cross_validate

[radlfabs]

Currently the cross validation loop iterates over base estimators..
With a flag we enter a second code block inside the loop where the mixed effects modeling takes place.
Therefore, LMER and MERF need to be treated differently as another model type as any other base estimator.
This is also due to the differences in API.
It would be benefitial to reduce this second part of the loop. MERF will always be in a different hierarchy because it is optimizing on top of base estimators. Unfortunately also LMER can not be base model inside MERF because MERF only supports fixed effects fitting internally for the base model.
A possible solution would be, to add a flag "fit_merf" to the ModelConfigDict, which triggers the part of the loop after fitting the base estimator to also enter into MERF fitting and evaluation.