Pixel area must be positive

[oczoske]

The METIS/IFU notebook used an input cube in which CDELT1 was negative. This resulted in a negative source trace imprinted on positive background. The reason is that OpticalTrain.prepare_source() divides the cube data by the pixel area (solid angle), which was computed from CDELT1 and CDELT2 and came out negative. This PR uses the absolute value of the pixel area so that the cube data remain positive.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.82%. Comparing base (67be391) to head (7b1d048).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
scopesim/optics/optical_train.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #878      +/-   ##
==========================================
- Coverage   74.83%   74.82%   -0.01%     
==========================================
  Files          69       69              
  Lines        8868     8869       +1     
==========================================
  Hits         6636     6636              
- Misses       2232     2233       +1     

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[teutoburg]

Always surprising to find such rather obvious things...