Supporting string choices for GPTuner

[TimSchim]

What would you like to be added: Currently the search space for GPTuner only supports numerical values for type choice. A possiblity to provide strings would be a convenient option.

Why is this needed: While it is possible to replace strings with numerical encodings, this reduces readability of code and intuitive interpretability of the hyperparameter plots.

Brief description of your proposal if any: Internal encoding to numerical values if strings are provided.

Duplicate of #1546