Enable inheritance from Prior class to the specialized priors

[daniel-saunders-phil]

follow up to #1939

That PR adds special_priors, which are a collection of objects that closely resembles the Prior class but with some differences. Right now, special priors don't inherit from the Prior class. But they should: they are required to have many of the same methods and inheritance is a good way to enforce that.

To get to inheritance, we'd need to refactor the Prior class over in the pymc-extras repo so there is a shared base class with abstract methods. Then we'd want to rewrite special priors to inherit from that base class.

[williambdean]

Why is inheritance needed?

[daniel-saunders-phil]

Not necessary for sure. The advantage I see is that a set of abstract methods would make it obvious what is expected for any future specialized prior object. You could inspect the base class to find out what methods you are supposed to write.

Maybe we could also save lines of code too because we can reuse some methods across classes if we abstract them a little bit. Unclear to me how unified the logic behind all these priors will end up because I don't know what other special priors we'd want.

As I explain it, it sounds like fairly marginal benefits 😅 what do you think Will?