added STEREO empirical model-based straylight

[s0larish]

• Added straylight model module in level0.py.
• The module generates both B and pB straylight data.
• These will be generated for WFI and NFI as required.

[codecov]

Codecov Report

Attention: Patch coverage is 86.02151% with 13 lines in your changes missing coverage. Please review.

Project coverage is 51.24%. Comparing base (620a6eb) to head (ec111dd).

Files with missing lines	Patch %	Lines
simpunch/level0.py	35.29%	11 Missing ⚠️
simpunch/util.py	93.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
- Coverage   53.73%   51.24%   -2.49%     
==========================================
  Files          10       12       +2     
  Lines         670      921     +251     
==========================================
+ Hits          360      472     +112     
- Misses        310      449     +139     

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[lowderchris]

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