Add basic support for IISs

[hedtke]

A first implementation that extracts the ids of the conflicting constraints.

Resolves #39

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.72%. Comparing base (6ab0c12) to head (8df6823).

Files with missing lines	Patch %	Lines
source/model.cpp	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
- Coverage   98.04%   97.72%   -0.33%     
==========================================
  Files          10       10              
  Lines         205      220      +15     
  Branches       57       62       +5     
==========================================
+ Hits          201      215      +14     
- Misses          3        4       +1     
  Partials        1        1              

Files with missing lines	Coverage Δ
include/scippp/model.hpp	96.00% <ø> (+0.16%)	⬆️
source/model.cpp	97.93% <92.85%> (-0.86%)	⬇️

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[hedtke]

I do not spot any problem with the coverage and would want to ignore that