Use unconstrain_array in CmdStan

[WardBrian]

Submisison Checklist

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

Summary:

Follow on to stan-dev/stan#3179. This replaces the array_var_context/transform_inits combo with a call to unconstrain_array.

It also makes some related cleanups to existing calls of transform_inits, since now (as of stan-dev/stanc3#1316), transform_inits now does validation on the var context it receives.

I also fixed up a couple small typos and some other minor things in docstrings.

Intended Effect:

Clean up various ways of unconstraining parameters, with the express goal being to make tuples easier to handle by delegating more to the compiler.

How to Verify:

Side Effects:

None

Documentation:

Existing docstrings updated as apropriate

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):

Simons Foundation

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!