Avoid symmetry complex-cast warnings

[galjos]

Summary

• convert symmetry-analysis coordinates to real floats before creating pymatgen molecules
• suppress the known benign pymatgen operations ComplexWarning inside point-group analysis
• reject atom positions with meaningful imaginary components
• add regression coverage for zero-imaginary and near-zero-imaginary positions

Validation

• python -m pytest tests/thermo/test_system.py -q
• python -m pytest -q
• python -m pytest --cov=ThermoScreening --cov-report=term-missing -q (94%)
• python -m pylint ThermoScreening
• Linux DFTB+ water thermochemistry with ComplexWarning treated as error

Closes #42

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.54%. Comparing base (2af48d0) to head (6705451).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   93.49%   93.54%   +0.05%     
==========================================
  Files          21       21              
  Lines        1168     1178      +10     
==========================================
+ Hits         1092     1102      +10     
  Misses         76       76              

Flag	Coverage Δ
unittests	93.54% <100.00%> (+0.05%)	⬆️

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