Add substitute_variables

[blegat]

• Change coefficients for offdiag vs diag quadratic terms

This also fixes a few bugs in MOIU.operate with quadratic terms

Extracted from #759

[codecov-io]

Codecov Report

Merging #811 into master will decrease coverage by 1.79%.
The diff coverage is 92.94%.

@@            Coverage Diff            @@
##           master     #811     +/-   ##
=========================================
- Coverage   94.12%   92.33%   -1.8%     
=========================================
  Files          61       65      +4     
  Lines        6914     7838    +924     
=========================================
+ Hits         6508     7237    +729     
- Misses        406      601    +195

Impacted Files	Coverage Δ
src/sets.jl	91.26% <ø> (-0.4%)	⬇️
src/Utilities/functions.jl	95.11% <92.94%> (-0.23%)	⬇️
src/Utilities/sets.jl	25% <0%> (-75%)	⬇️
src/Bridges/Constraint/indicator.jl	95.23% <0%> (-4.77%)	⬇️
src/Bridges/Constraint/geomean.jl	97.33% <0%> (-2.67%)	⬇️
src/Bridges/Constraint/functionize.jl	92.53% <0%> (-2.47%)	⬇️
src/Bridges/Constraint/rsoc.jl	97.56% <0%> (-2.44%)	⬇️
src/Bridges/Constraint/quad_to_soc.jl	95.55% <0%> (-1.78%)	⬇️
src/Bridges/Constraint/soc_to_psd.jl	98.8% <0%> (-1.2%)	⬇️
src/Bridges/Constraint/square.jl	97.36% <0%> (-0.91%)	⬇️
... and 12 more

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