`model_type` can override automatic choice of `predict_function` in Explainer

[hbaniecki]

if model_type is not None --> override guessing of classification/regression https://github.com/ModelOriented/DALEX/blob/daf33cbe54f0b624973722f4d208cb5edaa55cb4/python/dalex/dalex/_explainer/checks.py#L184