Consider refactoring of AgentMet4FoF to an abstract base class

[BjoernLudwigPTB]

The aforementioned class is actually not meant to be innstanciated directly, since it is missing some important implementations. In Python there are so-called abstract base classes (ABCs) to cover such use cases: https://docs.python.org/3/library/abc.html. We should consider making use of this feature and turning thhe accordings methods into @abstractmethod s.