Add a large scale illumination effect

[ShannonS00]

• New effect in effects/illumination.py that applies large-scale illumination variation (e.g. vignetting) to the image plane

2 Build in callable models:

1. _gaussian2d (normalised 2D elliptical Gaussian)
2. poly_vignetting (asymmetric polynomial with direct falloff control).

• Own model input possible
  Example:

  def my_model(xx, yy, slope=-0.001):
         return np.clip(1 + slope * np.sqrt(xx**2 + yy**2), 0, None)
  
     eff = Illumination(model=my_model, modelargs={"slope": -0.0005})
  

• can be included via !DET.include_illumination

Included Tests in scopesim/tests/tests_effects/test_illumination.py

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.19%. Comparing base (db7a81b) to head (6212521).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
scopesim/effects/illumination.py	86.44%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #898      +/-   ##
==========================================
+ Coverage   75.11%   75.19%   +0.07%     
==========================================
  Files          69       70       +1     
  Lines        8953     9013      +60     
==========================================
+ Hits         6725     6777      +52     
- Misses       2228     2236       +8     

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[oczoske]

Just one request about theta, otherwise that looks fine.

[oczoske]

Almost there...