Merge in Abstract bases, and remove SCHEDULER_BASE_CLASSES from docs?

[dwasyl]

Just taking a look at this tool, seems quite useful! However, I need to extend the Event with some extra fields and having the key models as abstract bases would be very useful here, I noticed the PR #488 which does exactly that. Any chance this could get merged in, or what would be required for it? The code looks like a fairly straight forward addition.

At the same time, it's probably a good idea to drop SCHEDULER_BASE_CLASSES from the docs since it's been gone for a few years.