Add XS_Jet model to xspec-models

[phajy]

Added the interface to XS_Jet, the jet model in XSPEC. Just realised I forgot the nice ASCII art overview plot to go with the model.

[fjebaker]

Thanks Andy! Request a review from me when you're ready to have it merged :)

[phajy]

Plot added. I'll request a review :)
EDIT: Don't think I can request a review directly, so will just have to request in the text here!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (56f0ba4) 66.90% compared to head (b00b6f5) 66.83%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
- Coverage   66.90%   66.83%   -0.07%     
==========================================
  Files          40       40              
  Lines        1988     1990       +2     
==========================================
  Hits         1330     1330              
- Misses        658      660       +2     

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[fjebaker]

LGTM!