Fix accidental mangling of model namespace name

[andrjohns]

Submission Checklist

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

Summary

Related to #1197, the model name argument is currently passed to stanc as a quoted string (i.e., stanc --name='bernoulli_model'), causing stanc to treat the quotes as part of the model name and escape/mangle them in the generated c++ (e.g., x39bernoulli_modelx39_namespace).

Additionally the pre-processing step for replacing spaces with underscores to generate the model name from filename is currently using the sub() function instead of gsub(), causing only the first space to be replaced (i.e., name with spaces.stan -> name_with spaces)

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/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.08%. Comparing base (d67fd53) to head (b9a0c28).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1200      +/-   ##
==========================================
- Coverage   91.10%   91.08%   -0.02%     
==========================================
  Files          15       15              
  Lines        6125     6127       +2     
==========================================
+ Hits         5580     5581       +1     
- Misses        545      546       +1     

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[jgabry]

Looks good! Will merge now