Feature/3247 json metric type - add field "metric_type" to metric.json file

[mitzimorris]

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

Add field "metric_type" with values "diag_e", "dense_e", and "unit_e" to json metric file.

Intended Effect

Make it easier to restart sampler - always specify metric type.

How to Verify

Unit tests will be added to CmdStan's tests of this feature once this PR is merged.

Side Effects

N/A

Documentation

N/A

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]

changes made; ready for re-review