Turn min-res-gen constraint into cuts and report constraint value

[manuelma]

Fixes #771

Checklist before merging

• Documentation is up-to-date
• Unit tests have been added/updated accordingly
• Code has been formatted according to SpineOpt's style
• Unit tests pass

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (8c4267f) 84.40% compared to head (9d211a6) 84.42%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
+ Coverage   84.40%   84.42%   +0.01%     
==========================================
  Files         147      147              
  Lines        3329     3319      -10     
==========================================
- Hits         2810     2802       -8     
+ Misses        519      517       -2     

Files	Coverage Δ
src/SpineOpt.jl	80.00% <ø> (ø)
src/constraints/constraint_mp_any_invested_cuts.jl	100.00% <ø> (ø)
.../constraint_mp_min_res_gen_to_demand_ratio_cuts.jl	100.00% <100.00%> (ø)
src/run_spineopt_benders.jl	95.52% <100.00%> (-0.23%)	⬇️
src/run_spineopt_mga.jl	94.11% <100.00%> (-0.62%)	⬇️
src/run_spineopt_standard.jl	85.11% <100.00%> (+0.23%)	⬆️

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