Enable save_cmdstan_config

[amas0]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

Add output of CmdStan config jsons to all models run through cmdstanpy. This initial PR enables the option, follow on PRs will read the output as part of our IO updates. Partially addresses #714.

• Adds save_cmdstan_config=1 to all CmdStan calls
• Add RunSet.config_files to keep track of output config JSONs
• Adds unit testing for new config saving

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): myself

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)