add function(s) to access parameter posteriors

[sbfnk]

          In a future issue we might want to add a R side version of this to help people find the posteriors for specific parameters (I may have missed this somewhere already)

Originally posted by @seabbs in #871 (comment)

[seabbs]

I think we might want to solicit some user feedback from power users (i.e. @zsusswein for example) to see if the new setup is making it hard for them to find anything they were previously using directly from the model object.

[zsusswein]

Thanks Sam! Would be happy to chat if I can be helpful.