fix(fits): ignore NaN values when fitting

[raimund-schluessler]

The Lorentz fit does not work if the spectrum contains NaN values. This can happen at the beginning or end of the spectrum and might lead e.g. to only one Rayleigh peak to be fitted.

Masking the NaN values fixes this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.10%. Comparing base (2cf8477) to head (52ad281).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
+ Coverage   88.08%   88.10%   +0.01%     
==========================================
  Files          22       22              
  Lines        2476     2479       +3     
==========================================
+ Hits         2181     2184       +3     
  Misses        295      295              

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