base recommender class (base.py) should be abstract

[hjwilli]

Indicate that base.py is abstract in some way, perhaps by using the abstract base class module 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)

[lacava]

we could/should add a NotImplementedError in recommend() (here) without a problem. However, most of the base methods are not abstract, they contain baseline shared functionality and are called via super() in derived classes (see here and here for example). We could rename these methods to make the parent methods abstract