recommender improvements: use namedtuple for storing ml/param/dataset combos

the use of [NamedTuple](https://docs.python.org/3/library/collections.html#collections.namedtuple) for storing combinations of datasets, ml, and parameters instead of strings joined with '|'
- this should make it easier to unpack the elements, use them together as keys in dicts if necessary, access them by name etc.