Parameter permutation

[AdrianSosic]

This PR adds code to sort the user-provided values before storing them as attributes in discrete parameters (see also #336).

If unsorted, this can cause problems with reproducibility in the sense that the same parameter content provided in a different order can lead to different optimization results. For instance, the RandomRecommender randomly selects rows from the comp_rep dataframe for the discrete subspace, which will be ordered differently if the parameter values come in a different order. This can lead to rather surprising behavior, like in situations when the parameter values are given as the output of Python's set function, whose order depends on PYTHONHASHSEED.