Add a new fit method - $bridge_sampler()

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Adds a new method to the CmdStanMCMC class for computing log marginal-likelihoods using the bridgesampling package

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[andrjohns]

@singmann @crsh I've taken an alternative approach to your PR and added a method for bridge sampling to cmdstanr objects directly.

Would you mind having a look when you've got a minute and letting me know if I've missed or mis-specified anything? Thanks!

[andrjohns]

Closing for now while we discuss implementation details internally