Move JSON parser from CmdStan to Stan/io

[WardBrian]

Submission Checklist

• [x] Run unit tests: ./runTests.py src/test/unit
• [x] Run cpplint: make cpplint
• [x] Declare copyright holder and open-source license: see below

Summary

This is the first half of #3142 and undoes #2803

Intended Effect

Move the JSON parser code from cmdstan to this repo. Tests are also moved.

How to Verify

Side Effects

Documentation

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):

Simons Foundation

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/)

[bob-carpenter]

I can review this when it's ready as long as that's today or early tomorrow or some time later this week.

[WardBrian]

@bob-carpenter this should be ready as-is; it's just a copy-paste job with some renaming of namespaces.

[WardBrian]

I'm going to let the CI here and https://github.com/stan-dev/cmdstan/pull/1120 finish before merging

[rok-cesnovar]

The upstream tests fail because the CmdStan PR was merged, but since all other tests pass feel free to merge.

[WardBrian]

@rok-cesnovar will that prevent the bot from updating the sub modules?

[rok-cesnovar]

I think it should be fine. The test that failed doesnt run on develop I believe.