Improve README and externalize DFTB parameters

[galjos]

Summary

• rewrite the README and move roadmap TODOs into issues
• require DFTB_PREFIX or slako_dir for Slater-Koster files
• remove bundled 3ob-3-1 parameter files and add pure unit coverage

Validation

• python -m pytest -q
• python -m pytest --cov=ThermoScreening --cov-report=term-missing -q (93%)
• python -m pylint ThermoScreening
• python -m pip wheel . --no-deps -w /tmp/thermoscreening-wheel-test

Follow-up issues: #29, #30, #32-#38

Closes #31

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.85%. Comparing base (b93b275) to head (9924dd2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   89.94%   92.85%   +2.91%     
==========================================
  Files          20       20              
  Lines        1074     1064      -10     
==========================================
+ Hits          966      988      +22     
+ Misses        108       76      -32     

Flag	Coverage Δ
unittests	92.85% <100.00%> (+2.91%)	⬆️

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