Hold a SCIP pointer to prevent freeing the model while the constraint is still in use

[mmghannam]

This is the second step for #151 that fixes use after free errors when using constraints after the model is freed.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.23%. Comparing base (d56c177) to head (a7b9ef3).
Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
src/model.rs	86.96%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
+ Coverage   75.67%   76.23%   +0.56%     
==========================================
  Files          13       13              
  Lines        1759     1805      +46     
==========================================
+ Hits         1331     1376      +45     
- Misses        428      429       +1     

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