Matpower Parser Fixes

[ccoffrin]

closes #729

[codecov]

Codecov Report

Merging #732 into master will increase coverage by 0.08%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master     #732      +/-   ##
==========================================
+ Coverage   92.70%   92.78%   +0.08%     
==========================================
  Files          42       42              
  Lines        7609     7792     +183     
==========================================
+ Hits         7054     7230     +176     
- Misses        555      562       +7     

Impacted Files	Coverage Δ
src/io/matpower.jl	92.46% <96.15%> (-0.04%)	⬇️
src/core/relaxation_scheme.jl	100.00% <0.00%> (ø)
src/form/lpac.jl	96.03% <0.00%> (+1.11%)	⬆️

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