Implement fixed_param version of sample method

[jgabry]

Implement fixed_param version of sample method

[dickoa]

Hi @jgabry
I hope all is good I wanted to know if this feature is still something that will be added to cmdstanr.
Thanks again

[rok-cesnovar]

Hi @dickoa,

fixed_param is in the works and will hopefully be done for the beta release.

[dickoa]

Hi @rok-cesnovar
Thanks a lot and great work on this awesome package!