Add broader thermochemistry benchmarks

[galjos]

Summary

• add checksum-pinned model-chemistry ZPE validation
• add a 29-molecule experimental entropy benchmark for DFTB+ and GFN2-xTB
• document sources, limits, and source-data exclusions

Validation

• 535 passed, 7 skipped; 96% coverage
• both external benchmarks pass
• docs, lint, and package build pass

Closes #140

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.82%. Comparing base (c0c0244) to head (08d51d9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #141   +/-   ##
=======================================
  Coverage   95.82%   95.82%           
=======================================
  Files          34       34           
  Lines        2877     2877           
=======================================
  Hits         2757     2757           
  Misses        120      120           

Flag	Coverage Δ
unittests	95.82% <ø> (ø)

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