Expose Core API `keep_selected_variables_only` to the Sklearn API

[popescu-v]

In v11, the Core API train_predictor parameter keep_selected_variables_only has been introduced. It is set to True by default, in which case only the selected variables in supervised analysis are kept in the preparation report.

However, in v10, all variables were present in the preparation report. In order to have all variables in the preparation report in Khiops v11 as well, we need to set keep_selected_variables_only to False in the train_predictor Core API call. Hence, we need to expose this parameter to the Sklearn API, as a KhiopsEstimator construction parameter.