Add CASEGEN_UPCARS forward model

[alifbe]

Add forward model description for CASEGEN_UPCARS and use logger to print out information/warning

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (ff349b9) 86.12% compared to head (a44d358) 86.13%.

❗ Current head a44d358 differs from pull request most recent head 3fe6977. Consider uploading reports for the commit 3fe6977 to get more accurate results

Files	Patch %	Lines
src/subscript/casegen_upcars/casegen_upcars.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #649   +/-   ##
=======================================
  Coverage   86.12%   86.13%           
=======================================
  Files          50       50           
  Lines        7065     7069    +4     
=======================================
+ Hits         6085     6089    +4     
  Misses        980      980           

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[mferrera]

Aside from Roger's comment the code LGTM 👍