Revisit handling of default priors

[seabbs]

Default priors are currently handled by small helper functions that have custom names. This seems surprisingly clunky given the rest of the code base.

Whilst I agree default priors in structs could be dangerous in the hands of some users I also think providing opinionated priors can be very helpful and also guide practice.

Personally I would vote for default priors in the structs but open to other suggestions?

[SamuelBrand1]

My view is that if we can close #133 in such a way that all priors have defaults for their constructor; then this is closed too.

[seabbs]

I think this is now resolved? (checked and yes closed by #162)