Feature/3195 json writer move ctor

[mitzimorris]

Submission Checklist

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

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[mitzimorris]

@SteveBronder - with this in place, we can add method 'pathfinder' to CmdStan.

[SteveBronder]

lgtm!