Bugfix/2346 standalone functions for rstan

[martinmodrak]

Submisison Checklist

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

Summary

With respect to #2346 - fix problems in standalone function compilation that block integration with RStan.
As a part of this effort, added a new integration test that checks that all src/test/test-models/good-standalone-functions/*.stanfuncs files succesfully compile.

Intended Effect

RCpp export only for non-templated functions, removed using of lgamma.

How to Verify

Test with RStan :-) For the lgamma, a new test checking whether lgamma compiles in a standalone func has been added.

Side Effects

Hopefully none.

Documentation

N/A

Reviewer Suggestions

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): Martin Černý / Institute of Microbiology of the Czech Academy of Sciences

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/)