AxisId as single indice of a set of variables/constraints

[guimarqu]

fix for atoptima/Coluna.jl#539 (comment)

[codecov]

Codecov Report

Merging #60 (8565893) into master (ad363c8) will increase coverage by 0.56%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   75.63%   76.20%   +0.56%     
==========================================
  Files          11       11              
  Lines         394      395       +1     
==========================================
+ Hits          298      301       +3     
+ Misses         96       94       -2     

Impacted Files	Coverage Δ
src/axis.jl	88.46% <100.00%> (+4.14%)	⬆️

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