Turn warnings into errors in CI and fix last remaining warnings

[blegat]

Closes #385

[codecov-io]

Codecov Report

Merging #393 into master will increase coverage by 0.02%.
The diff coverage is 95.45%.

@@            Coverage Diff            @@
##           master    #393      +/-   ##
=========================================
+ Coverage   96.47%   96.5%   +0.02%     
=========================================
  Files          39      39              
  Lines        4914    4915       +1     
=========================================
+ Hits         4741    4743       +2     
+ Misses        173     172       -1

Impacted Files	Coverage Δ
src/Test/contconic.jl	100% <ø> (ø)	⬆️
src/Bridges/Bridges.jl	100% <ø> (ø)	⬆️
src/Utilities/mockoptimizer.jl	97.46% <ø> (-0.02%)	⬇️
src/constraints.jl	80% <100%> (ø)	⬆️
src/functions.jl	100% <100%> (ø)	⬆️
src/Test/UnitTests/constraints.jl	100% <100%> (ø)	⬆️
src/indextypes.jl	62.5% <100%> (ø)	⬆️
src/Bridges/bridgeoptimizer.jl	98.93% <100%> (ø)	⬆️
src/Bridges/soctopsdbridge.jl	100% <100%> (ø)	⬆️
src/Test/modellike.jl	100% <100%> (ø)	⬆️
... and 7 more

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