Add DFTB setup helper commands

[galjos]

Summary

• add setup-dftb to install the default 3ob Slater-Koster files outside the repo
• add doctor diagnostics for DFTB+ executables and DFTB_PREFIX
• document the workflow and exclude tests from release wheels

Validation

• python -m pytest -q
• python -m pytest --cov=ThermoScreening --cov-report=term-missing -q (94%)
• python -m pylint ThermoScreening
• python -m pip wheel . --no-deps -w /tmp/thermoscreening-wheel-test

Closes #40

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.49%. Comparing base (4ed5990) to head (ed223d9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
+ Coverage   92.85%   93.49%   +0.63%     
==========================================
  Files          20       21       +1     
  Lines        1064     1168     +104     
==========================================
+ Hits          988     1092     +104     
  Misses         76       76              

Flag	Coverage Δ
unittests	93.49% <100.00%> (+0.63%)	⬆️

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