qualiative descriptions should be configurable

[Schaechtle]

FROM PR review:

the internal function _prefix_category_name is widely used by public functions but makes assumptions about what the caller wants that is not configurable -- namely, the use of _QUALITATIVE_LABELS. I am skeptical that every time you want to call the public function get_quantile_based_discretization_function that you can assume it is desirable to include names like low and high, which are descriptive.