added a convergence integration test.

[rozyczko]

This pull request adds a new integration test suite to verify the convergence behavior of LMFit minimizers on real and synthetic reflectometry data.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.13%. Comparing base (2febb08) to head (2c64b86).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #436   +/-   ##
========================================
  Coverage    95.13%   95.13%           
========================================
  Files           56       56           
  Lines         6249     6249           
========================================
  Hits          5945     5945           
  Misses         304      304           

Flag	Coverage Δ
integration	40.27% <ø> (+1.98%)	⬆️
unittests	95.13% <ø> (ø)

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