Update base classes for code coverages

[ominusliticus]

There is no way to construct an abstract base class in python. Therefore, there is no way to have code coverage check the lines that have return NotImplemented. Simply have a doc-string (or pass if there is no doc-string) is enough.

[ominusliticus]

I will add here, as a comment for the permanent record, that our abs tract methods should have return NotImplementedError, instead of NotImplemented. However, reading the Python documentation, it seems the decorating the methods with @abtractmethod does this for us by requiring inheritors of the Abstract Base Class (ABC), to implemented all functions decorated as abstract.