Add constants for common defaults such as IID(Normal())

[seabbs]

          The default version of this is so common, would it be worth having a constant like

const NormalIID = IID()

Which can be used in places like the generate_latent method for HierarchicalNormal?

Originally posted by @SamuelBrand1 in #438 (comment)

[SamuelBrand1]

Yes. I'm abit agnostic, but I personally like const s for really commonly used objects. ymmv though.