Add error propagation estimates for BoxcarExtract and HorneExtract

[harry353]

This changes how uncertainties are handled in:

• BoxcarExtract.__call__
• HorneExtract.__call__
• HorneExtract._fit_gaussian_spatial_profile

as described in #282. Now they:

• Work correctly with both standard deviation and variance uncertainties.
• Pass the right uncertainty through to the extracted 1D spectrum.
• Keep units consistent.
• Handle NaNs and masked pixels better when fitting the Gaussian profile.

Also, I have added test scripts under the tests directory, as well as a .ipynb file for the Horne case with some examples.

[harry353]

Also currently working on issue #281 as well

[codecov]

Codecov Report

❌ Patch coverage is 75.75758% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.22%. Comparing base (0cfc970) to head (0f85d34).

Files with missing lines	Patch %	Lines
specreduce/extract.py	75.75%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #286      +/-   ##
==========================================
- Coverage   87.11%   86.22%   -0.90%     
==========================================
  Files          15       15              
  Lines        1281     1394     +113     
==========================================
+ Hits         1116     1202      +86     
- Misses        165      192      +27     

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