change optimize.minimize method from SLSQP to Nelder-Mead

[veni-vidi-vici-dormivi]

Changing the method in optimize.minimize` to Nelder-Mead from SLSQP should make the coefficients fit more stable, see #440 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.19%. Comparing base (9b0b76b) to head (ff8f733).
Report is 84 commits behind head on main.

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HEAD has 2 uploads less than BASE

Flag	BASE (9b0b76b)	HEAD (ff8f733)
unittests	3	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #455      +/-   ##
==========================================
- Coverage   87.90%   82.19%   -5.72%     
==========================================
  Files          40       43       +3     
  Lines        1745     1938     +193     
==========================================
+ Hits         1534     1593      +59     
- Misses        211      345     +134     

Flag	Coverage Δ
unittests	82.19% <ø> (-5.72%)	⬇️

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