allow to load MATPOWER instances directly from PGLIB/MATPOWER

[frapac]

• add PGLIB benchmark in Artifacts.toml
• add function PS.load_case to load PowerNetwork from case's name
• add function load_polar to load PolarForm from case's name

[codecov-commenter]

Codecov Report

Merging #257 (63c02f5) into master (ce40761) will decrease coverage by 0.05%. The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
- Coverage   75.66%   75.61%   -0.06%     
==========================================
  Files          29       28       -1     
  Lines        3468     3473       +5     
==========================================
+ Hits         2624     2626       +2     
- Misses        844      847       +3     

Impacted Files	Coverage Δ
src/PowerSystem/PowerSystem.jl	97.87% <ø> (-0.05%)	:arrow_down:
src/PowerSystem/power_network.jl	92.14% <66.66%> (-1.30%)	:arrow_down:
src/Polar/polar.jl	100.00% <100.00%> (ø)
src/LinearSolvers/LinearSolvers.jl	75.75% <0.00%> (-0.37%)	:arrow_down:
src/PowerSystem/parsers/parse_mat.jl	25.94% <0.00%> (-0.13%)	:arrow_down:
src/PowerSystem/parsers/parse_psse.jl	100.00% <0.00%> (ø)
src/ExaPF.jl

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[frapac]

I agree. But I think the LazyArtifacts is not straightforward for new user. With this utils functions, we have:

1. We can import any datafile with a syntax close to MATPOWER
2. And we have access to the PGLIB benchmark, in addition to the instances stored in ExaData