Move retrieval implementation into main app folder

The retrieval implementation should be promoted from mere experiment to a part of the main aurora application, i.e., the aurora folder. To do so:

• Move all necessary requirements to main requirements.txt
• Split experiment code up into as many classes as necessary
• Move every class to its own Python file somewhere within aurora.knowledge
• If there are longer functions in the code that don't belong to a class, also move them into a respective Python file containing only that function
• Add all relevant configuration (e.g. paths to data) as properties to Config
• Implement returning a KnowledgeBase instance in Config

To get an idea of how such productive app code look like, please check out !12 (merged)