Added set_obj_integral Method

[CodingTil]

See Issue #95

cargo build runs through, as well as cargo fmt (at least for my code).
I am unable to run cargo test at the moment, it currently says it cannot find libscip.8.0.dylib - seems unrelated to my code.

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Changelog:

• Added method set_obj_integral in scip.rs which calls SCIPsetObjIntegral
• Added method set_obj_integral to Model<ProblemCreated> to allow specifying that the objective value is always integral
• Added a very simple test which simply verifies that the method does not raise any error

[CodingTil]

Update: Managed to get cargo test working. Test fails: [scip_prob.c:1547] ERROR: method is not callable in SCIP stage <0>
I think see https://github.com/scipopt/scip/blob/beae291be2e4daf8cf581c2b703bd38c53dd5dcf/src/scip/scip_prob.c#L1509 - one cannot call it at the Unsolved Stage?

[codecov]

Codecov Report

Merging #101 (093957e) into main (1ffb0d1) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
+ Coverage   91.29%   91.36%   +0.06%     
==========================================
  Files          13       13              
  Lines         804      810       +6     
==========================================
+ Hits          734      740       +6     
  Misses         70       70              

Files Changed	Coverage Δ
src/model.rs	97.86% <100.00%> (+0.03%)	⬆️
src/scip.rs	98.21% <100.00%> (+0.01%)	⬆️

[mmghannam]

Looks good! Thank you for the useful contribution. Just one minor change request and would be ready to merge, please don't forget to update the changelog.

[CodingTil]

please don't forget to update the changelog

Oh, I realize now you meant CHANGELOG.md - will do that

[mmghannam]

Perfect, thanks again @CodingTil! Will merge right after the pipelines pass, looking forward to your next contribution :)