Refactor KB for easier customization

[rmitsch]

Goal

Refactor the existing KnowledgeBase class to facilitate easier customization and do away with the tight coupling between KB interface and its internal implementation.

Note: Documentation updates and additional tests are still missing and will be added post-review.

Description

• An (abstract) base class for KnowledgeBase derivatives.
• Backwards-compatible adjustments to the existing KnowledgeBase implementation to adhere to the new interface,
• Consideration of batched candidate generation/lookup routines.

Types of change

Checklist

• [x] I confirm that I have the right to submit this contribution under the project's MIT license.
• [ ] I ran the tests, and all new and existing tests passed.
• [ ] My changes don't require a change to the documentation, or if they do, I've added all required information.