change treatment of string features in metalearning (#1426)
change treatment of string features in metalearning (#1426)

* change treatment of string features. In file `smbo.py` string columns are marked as categorical. Previously string columns are treated as not categorical which transfers to numerical. We decided that encoded string columns are more similar to OHE as they are to numerical transformations. This change effects the metalearning part of `autosklearn` exclusively.

* change treatment of string features. In file `smbo.py` string columns are marked as categorical. Previously string columns are treated as not categorical which transfers to numerical. We decided that encoded string columns are more similar to OHE as they are to numerical transformations. This change effects the metalearning part of `autosklearn` exclusively.
automl/auto-sklearn,automl/auto-sklearn