Add method to get posterior samples from the posterior chains more easily

[odunbar]

For plotting one often wishes to just have an array, rather than the chains object in the posterior. It would be good to pull this out more conveniently

Current implementation

# given: posterior = get_posterior(mcmc, chain)
posterior_samples = vcat([get_distribution(posterior)[name] for name in get_name(posterior)]...) #samples are columns
constrained_posterior_samples =
    mapslices(x -> transform_unconstrained_to_constrained(posterior, x), posterior_samples, dims = 1)

Solution:

Firstly, we don't need mapslices as

transform_unconstrained_to_constrained(posterior, posterior_samples)

should work? see here

Perhaps we can just wrap the first part up like:

posterior_samples = get_posterior(mcmc, chain, dist_or_array="array") # add to CES.jl?
# or 
posterior_sampels = get_samples_as_array(posterior)

PS we should also avoid vcat(X...) where possible