Description

Give a warning if model is already fitted.
This is acceptable in case of custom experimentation, but not desirable if we want to upload to OpenML. For transparent interpretation of OpenML results, the user should be told of what's happening.

Can be checked for in the _run_model_on_fold() function in openml.extensions.sklearn.extension prior to calling of model_copy.fit().

Possible solutions:

• https://stackoverflow.com/a/39900933/8363967
• https://scikit-learn.org/stable/modules/generated/sklearn.utils.validation.check_is_fitted.html