More tar_stan_mcmc_rep*() functions

[wlandau]

Prework

• [x] I understand and agree to the code of conduct and contributing guidelines.
• [x] If there is already a relevant issue, whether open or closed, comment on the existing thread instead of posting a new issue.
• [x] New features take time and effort to create, and they take even more effort to maintain. So if the purpose of the feature is to resolve a struggle you are encountering personally, please consider first posting a "trouble" or "other" issue so we can discuss your use case and search for existing solutions first.

Proposal

We currently have tar_stan_mcmc_rep_summary() to get multiple $summary()s, but users may want to return the actual draws or sampler diagnostics from simulation studies.