base recommender class (base.py) should be abstract

Indicate that base.py is abstract in some way, perhaps by using the [abstract base class module](https://docs.python.org/3/library/abc.html) or raising a `NotImplementedError` if a base instance is instantiated.

(see https://stackoverflow.com/questions/13646245/is-it-possible-to-make-abstract-classes-in-python)