For laplace, don't overwrite optimize CSV when mode=NULL and output_basename is specified

[jgabry]

Submission Checklist

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

Summary

For the laplace method, in the specific case where mode=NULL and output_basename is specified, the CSV written by optimize could be overwritten. This gives it a unique name (adding mode to the name) so it can't be overwritten.

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):
Jonah Gabry

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 90.44%. Comparing base (ddf749b) to head (4baab8c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1191   +/-   ##
=======================================
  Coverage   90.44%   90.44%           
=======================================
  Files          15       15           
  Lines        6124     6125    +1     
=======================================
+ Hits         5539     5540    +1     
  Misses        585      585           

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