Simplify SDP index mapping

[blegat]

I was a bit worried at first when reading it because mapindices create columnwise lower-triangular indices while the MOI convention is columnwise upper-triangular. Then it seems that the way it's used just goes back and forth and at it end it just gives the MOI variable indices ? I might be wrong but let's try (I need to get in the plane so no time to test :laughing: )

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.30 :warning:

Comparison is base (df1d1f1) 82.26% compared to head (6b22de0) 81.97%.

:exclamation: Current head 6b22de0 differs from pull request most recent head b80a965. Consider uploading reports for the commit b80a965 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
- Coverage   82.26%   81.97%   -0.30%     
==========================================
  Files          12       12              
  Lines        1269     1254      -15     
==========================================
- Hits         1044     1028      -16     
- Misses        225      226       +1     

Impacted Files	Coverage Δ
src/structs.jl	80.00% <ø> (ø)
src/MOI_wrapper.jl	93.95% <100.00%> (-0.06%)	:arrow_down:
src/scaling.jl	95.65% <100.00%> (-0.96%)	:arrow_down:

... and 1 file with indirect coverage changes

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[blegat]

The error seems unrelated since I get it in https://github.com/mariohsouto/ProxSDP.jl/pull/99 as well

[joaquimg]

looks good, just have to fix conflicts