Replace named iterator for priors with Dict objects

[SamuelBrand1]

Current API

structs with information about prior(s) currently hold those as NamedTuple objects.

Problem

Although, using NamedTuples is convenient; they are annoying to strongly specify their type. However, following #65 we now will error if the object giving distribution information about priors cannot be parsed as a Symbol => D where D <: Distribution pair.

Solution

Rather than holding information on priors as NamedTuples, replace with Dict{Symbol,D} in parameteric typed struct (e.g. where D <: Distribution.