Add MATPOWER data to Artifacts.toml

[sshin23]

We should also allow the user to use data from MATPOWER repo https://github.com/MATPOWER/matpower

[sshin23]

MATPOWER data testing is missing. Currently, it throws an error:

julia> parse_matpower("case118.m"; library = :matpower);
ERROR: BoundsError: attempt to access empty SubString{String} at index [1]
Stacktrace:
 [1] checkbounds
   @ ./strings/basic.jl:216 [inlined]
 [2] getindex
   @ ./strings/substring.jl:100 [inlined]
 [3] get_arr_len(lines::Vector{SubString{String}}, num_lines::Int64, start::Int64)
   @ ExaPowerIO ~/git/ExaPowerIO.jl/src/parser.jl:347
 [4] parse_matpower_inner
   @ ~/git/ExaPowerIO.jl/src/parser.jl:526 [inlined]
 [5] parse_matpower(::Type{Float64}, ::Type{Vector}, path::String; library::Symbol)
   @ ExaPowerIO ~/git/ExaPowerIO.jl/src/ExaPowerIO.jl:36
 [6] parse_matpower
   @ ~/git/ExaPowerIO.jl/src/ExaPowerIO.jl:22 [inlined]
 [7] #parse_matpower#17
   @ ~/git/ExaPowerIO.jl/src/ExaPowerIO.jl:39 [inlined]
 [8] top-level scope
   @ REPL[4]:1

[hfytr]

It looks like matpower puts matlab code in the .m files to perform conversions. they do this the same way in every file, so i could try to hack around it. technically matpower files are just matlab, so the parser wont be correct.

[hfytr]

also, im getting an error when trying to parse matpower cases (specifically tried with case10ba) with PowerModels.jl. is this just me?