Format double arguments with std::stringstream

[WardBrian]

Submisison Checklist

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

Summary:

Fixes #1242

Intended Effect:

The example in #1242 now prints:

$ ./bernoulli optimize algorithm=lbfgs tol_obj=1e-11 data file=bernoulli.data.json
method = optimize
  optimize
    algorithm = lbfgs (Default)
      lbfgs
        init_alpha = 0.001 (Default)
        tol_obj = 1e-11

How to Verify:

Run the above

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/)