Fix: issue with GQ arg checks, const ref string passing

[WardBrian]

Submisison Checklist

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

Summary:

This fixes a couple issues which came up as warnings for me:

• the argument checking for GQ had a mismatched if-else which lead to one of the conditionals never being checked
• a few places we were passing std::strings by value where a const ref would work

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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]

lgtm! thanks for the cleanup