Fix SOC to PSD bridge

[blegat]

Closes #838
@chriscoey Also adds operate_output_index! methods you need for #818

[codecov-io]

Codecov Report

Merging #840 into master will not change coverage.
The diff coverage is 97.72%.

@@           Coverage Diff           @@
##           master     #840   +/-   ##
=======================================
  Coverage   94.83%   94.83%           
=======================================
  Files          71       71           
  Lines        7649     7649           
=======================================
  Hits         7254     7254           
  Misses        395      395

Impacted Files	Coverage Δ
src/Bridges/Constraint/soc_to_psd.jl	100% <100%> (ø)	⬆️
src/Utilities/functions.jl	94.63% <80%> (+0.04%)	⬆️

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[chriscoey]

Thanks. After you merge this, I'll update #818.