Update implementations with add_constraint_variable(s)

[blegat]

Extracted from #759

[codecov-io]

Codecov Report

Merging #802 into master will increase coverage by <.01%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master     #802      +/-   ##
==========================================
+ Coverage   93.97%   93.97%   +<.01%     
==========================================
  Files          59       59              
  Lines        6690     6794     +104     
==========================================
+ Hits         6287     6385      +98     
- Misses        403      409       +6

Impacted Files	Coverage Δ
src/Utilities/copy.jl	90.64% <90.47%> (-2.29%)	⬇️
src/variables.jl	62.5% <0%> (+37.5%)	⬆️

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