Assume relative paths in cantera.conf are relative to the source root

[bryanwweber]

Changes proposed in this pull request

• Assume that relative paths passed on the command line or in cantera.conf are relative to the source root. Don't change absolute paths.
• I verified that this works locally on Windows and Linux. I also verified that absolute paths on Windows still work correctly.

If applicable, fill in the issue number this pull request is fixing

Closes #1743

If applicable, provide an example illustrating new features this pull request is introducing

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.12%. Comparing base (f1b98ee) to head (573dbf1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1750      +/-   ##
==========================================
- Coverage   73.13%   73.12%   -0.01%     
==========================================
  Files         381      381              
  Lines       54134    54134              
  Branches     9222     9222              
==========================================
- Hits        39590    39587       -3     
- Misses      11587    11589       +2     
- Partials     2957     2958       +1     

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[ischoegl]

Thanks, @bryanwweber ... this all looks good to me!