Switch project license to LGPL

[galjos]

Summary

• replace the MIT license with LGPL v2.1-or-later text
• update package metadata and README license note

Validation

• python -m pytest -q
• python -m pylint ThermoScreening
• python -m pip wheel . --no-deps -w /tmp/thermoscreening-wheel-test

Closes #20

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.94%. Comparing base (c7af1c3) to head (9da8562).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #28   +/-   ##
=======================================
  Coverage   89.94%   89.94%           
=======================================
  Files          20       20           
  Lines        1074     1074           
=======================================
  Hits          966      966           
  Misses        108      108           

Flag	Coverage Δ
unittests	89.94% <ø> (ø)

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