Add prior_summary() function and what_priors()

[wwiecek]

We can have two ideas:

• what_priors("rubin") or what_priors("sslab"). This would print out names of allowed priors and explain what they do
• prior_summary(fitted_model) as in rstanarm, which then tell us what values of priors have been set
• get_prior(fitted_model) to grab prior from one baggr model and reuse in another

Also have a look here: https://mc-stan.org/rstanarm/reference/prior_summary.stanreg.html