Use the new get_param_names bool arguments in initialize

[SteveBronder]

Submission Checklist

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

Summary

Uses stan-dev/stanc3#1241 to get back only the parameter names when setting initial values, Fixes stan-dev/stanc3#1240 so that when the user specifies initial values they are only run once.

This PR will fail until stan-dev/stanc3#1241 is merged

Intended Effect

Fixes stan-dev/stanc3#1240

How to Verify

Do I need additional tests for this? I'm not sure what kind of test we would want.

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): Steve Bronder

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/)

[WardBrian]

Oh, I put together #3139 to do the same thing.

These need to be arguments to the existing function, not a new overload, or else model.get_param_names(vec) becomes ambiguous and fails to compile

[WardBrian]

Subsumed by #3139