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.

[seabbs]

@zsusswein any feedback you can give here on interacting with the new interface would be useful. Our main concern is users wanting access to estimated parameters that are now part of the generic parameter vector used behind the scenes for the new prior interface.