Restructure prior module

The prior.py module is pushing towards 4000 lines of code since we had some recent additions (Joint priors, conditional priors). I suggest splitting this up into separate modules while maintaining backwards compatibility. I am happy to have a look into this soon.

changed milestone to %1.0.0

added Priors Refactoring labels

I've been thinking this for a while too. I had a brief look at it at one stage. I think I ran into issues with circular dependencies which aren't obvious when everything is in one module.

created merge request !688 (merged) to address this issue

mentioned in merge request !688 (merged)

closed via merge request !688 (merged)

mentioned in commit e567db4c