Fix handling of save_single_paths argument for pathfinder

[jgabry]

Submission Checklist

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

Summary

Previously save_single_paths = TRUE could be ignored silently.

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 91.23%. Comparing base (5576fd7) to head (1b589eb).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1192      +/-   ##
==========================================
+ Coverage   91.21%   91.23%   +0.01%     
==========================================
  Files          15       15              
  Lines        6125     6125              
==========================================
+ Hits         5587     5588       +1     
+ Misses        538      537       -1     

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