Refactor core module to use dependency inversion principle

[fabiangeisler]

• pyshotgrid version: v1.0.1

Description

Currently the core module is a giant monolith which I build, because I ran into massive import loop issues. Turns out by using the dependency inversion principle this could be avoided and the code could be reorganised again. I would like to implement it for readability alone but I can image this will have some other benefits too.

[fabiangeisler]

Turns out it is not as easy as I thought it would be. Since we have a somewhat bidirectional relation ship between the classes the interface approach kind of falls apart. Need to put more research into this.