Feature/1192 save metric json

[mitzimorris]

Submisison Checklist

• Run tests: ./runCmdStanTests.py src/test
• Declare copyright holder and open-source license: see below

Summary:

Add option to adaptive sampler args to save metric in separate JSON file with elements "stepsize" and "inv_metric"

Intended Effect:

Easy way to get pre-specified metric.

How to Verify:

Unit tests will pass once stan-dev/stan#3230 has been merged.

Side Effects:

N/A

Documentation:

Will add separately to docs repo.

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): Columbia University

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/)

[mitzimorris]

this is ready for re-review

[WardBrian]

This is looking really nice. A few comments but they're all minor now

[mitzimorris]

one mo' time?