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