Stabilize thermochemistry workflow

[galjos]

Summary

• Fix source-tree version lookup and package entry point.
• Implement .gen coordinate parsing and harden input parsing.
• Improve DFTB+ path handling and executable checks.
• Trim unused dependencies and add focused regressions.

Validation

• python -m pytest --cov=ThermoScreening --cov-report=term -q -> 40 passed
• DFTB+ integration tests -> included in the full run
• Clean Linux container with DFTB+ and coverage -> 40 passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.01%. Comparing base (b21764a) to head (41eefd1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
+ Coverage   82.47%   87.01%   +4.54%     
==========================================
  Files          19       20       +1     
  Lines        1044     1063      +19     
==========================================
+ Hits          861      925      +64     
+ Misses        183      138      -45     

Flag	Coverage Δ
unittests	87.01% <100.00%> (+4.54%)	⬆️

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