Provide abstract base classes for handlers

[daniel-j-h]

We have two use-cases for handlers:

• rs extract where handlers have to walk over OpenStreetMap gathering polygons
• rs features where handlers have to do post-processing on segmentation masks

We should provide abstract base classes for both use-cases making the interface explicit.

Docs: https://docs.python.org/3/library/abc.html#abc.abstractmethod

Tasks

• [ ] rs extract - write handler abstract base class
• [ ] rs features - write handler abstract base class