Allow executing cmdstan through WSL

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

This PR adds the ability to install cmdstan and run all models through WSL, all from a regular Windows R session.

The end-user simply needs to add wsl=TRUE to the install_cmdstan() call, and all model compilation and execution for that installation will subsequently be run through WSL. No other user-facing changes are made.

In addition to the known speed differences between Stan models under Windows & WSL, this also provides an accessible means for running MPI models in Windows

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):
Andrew Johnson

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/)

[andrjohns]

Forgot to mention in the description - this PR also adds a Github actions workflow to run the unit tests under WSL as well

[rok-cesnovar]

WOW! This is awesome stuff and will be a huge benefit to Windows users! Fantastic!

I am going to review this later today and tomorrow in detail, just a few questions so I know I am understanding this correctly:

• you can execute any command in WSL by running "wsl something.exe", is that correct? Has this been the case always?
• The install location remains the same, but installs the WSL CmdStan's with a wsl- prefix?
• WSL is used in a fresh session if the selected CmdStan installation path starts with wsl- or CMDSTANR_USE_WSL is set?

Lastly, please add yourself to the list of contributors.

[codecov-commenter]

Codecov Report

Merging #677 (9f4e8a6) into master (85a504d) will decrease coverage by 1.92%.
The diff coverage is 48.38%.

@@            Coverage Diff             @@
##           master     #677      +/-   ##
==========================================
- Coverage   91.86%   89.94%   -1.93%     
==========================================
  Files          12       12              
  Lines        3540     3649     +109     
==========================================
+ Hits         3252     3282      +30     
- Misses        288      367      +79     

Impacted Files	Coverage Δ
R/csv.R	97.66% <16.66%> (-1.05%)	⬇️
R/utils.R	79.74% <31.57%> (-9.37%)	⬇️
R/install.R	62.93% <34.00%> (-4.84%)	⬇️
R/path.R	71.05% <35.71%> (-8.32%)	⬇️
R/run.R	95.26% <70.58%> (-0.72%)	⬇️
R/args.R	98.65% <87.50%> (-0.19%)	⬇️
R/model.R	91.22% <95.45%> (+0.05%)	⬆️

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[rok-cesnovar]

I went ahead and just reviewed. Ignore my questions above, I think I got all the answers from the code. The code change looks great and the tests are passing in all OSes. I just want to take a few minutes to test it live on my Windows machine before final approval.

I just have two comments, both are very minor and optional.

[rok-cesnovar]

Thanks again! Will update NEWS.md and bump the version to 0.5.3.