add get_cmdstan_args method

[avehtari]

Closes #1133

Finding the CmdStan default arguments for different methods from the documentation is not easy, and many arguments are named differently in CmdStan and in CmdStanR. New model method $get_cmdstan_args() gets the defaults calling CmdStan itself and maps the argument names to those used by CmdStanR,

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):

Aki Vehtari

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/)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.09091% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.29%. Comparing base (5809552) to head (7b45182).

Files with missing lines	Patch %	Lines
R/model.R	99.09%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1167      +/-   ##
==========================================
- Coverage   90.85%   90.29%   -0.56%     
==========================================
  Files          14       14              
  Lines        5924     6027     +103     
==========================================
+ Hits         5382     5442      +60     
- Misses        542      585      +43     

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