Id subtype of Integer

[guimarqu]

So we can use them as indices of Julia sparse arrays.

[codecov]

Codecov Report

Merging #709 (3a380ed) into master (5ecd32d) will increase coverage by 0.33%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
+ Coverage   85.96%   86.30%   +0.33%     
==========================================
  Files          58       58              
  Lines        5702     5703       +1     
==========================================
+ Hits         4902     4922      +20     
+ Misses        800      781      -19     

Impacted Files	Coverage Δ
src/MathProg/formulation.jl	81.98% <ø> (+0.52%)	⬆️
src/MathProg/vcids.jl	90.00% <90.00%> (+7.24%)	⬆️
src/Algorithm/basic/solvelpform.jl	88.23% <0.00%> (-2.95%)	⬇️
src/MathProg/varconstr.jl	94.95% <0.00%> (+0.84%)	⬆️
src/Algorithm/benders.jl	89.14% <0.00%> (+0.98%)	⬆️
src/Algorithm/preprocessing.jl	76.87% <0.00%> (+1.13%)	⬆️
src/Algorithm/conquer.jl	80.37% <0.00%> (+1.26%)	⬆️
src/ColunaBase/nestedenum.jl	90.72% <0.00%> (+2.06%)	⬆️
src/Algorithm/branching/scores.jl	92.68% <0.00%> (+3.65%)	⬆️

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