add support for cardinality constraints

[yonch]

This adds support for SCIP cardinality constraints, which constrain solutions to have at most N non-zeros out of a set of variables (see https://www.scipopt.org/doc/html/cons__cardinality_8h.php).

This follows the others add_cons_* implementations in the crate, and adds a (passing) test.

scip-sys already has the relevant functions wrapped, so this is ready to be merged.

A follow up pull request to good_lp relies on this PR, so if possible, maintainers should consider making a named release, for good_lp to depend on.

[codecov]

Codecov Report

Merging #106 (3d93a06) into main (667ad6c) will increase coverage by 0.20%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   75.01%   75.22%   +0.20%     
==========================================
  Files          13       13              
  Lines        1669     1707      +38     
==========================================
+ Hits         1252     1284      +32     
- Misses        417      423       +6     

Files Changed	Coverage Δ
src/scip.rs	84.62% <66.67%> (-0.63%)	⬇️
src/model.rs	92.64% <100.00%> (+0.37%)	⬆️

[mmghannam]

Thank you for the useful contribution! I added some small comments, otherwise would be ready to merge.