Separate Core into separate files

[rclough]

This has been bugging me for a while. Core is like >1000 lines long, and there's a lot of repetitive term usage across the classes, which makes searching for functions annoying.... I think it's time we just put each class in its own file.

I'll probably do this after the current PR I'm working on. Should be quick? Unless theres some python package magic that I'm not aware of that makes this particularly difficult.

[thieman]

+1

[rclough]

Also taking this opportunity to conform core to PEP8 #82