Add test for dcline pwl costs

[ccoffrin]

closes #274

[codecov]

Codecov Report

Merging #877 (59437f7) into master (27b564e) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #877      +/-   ##
==========================================
+ Coverage   93.86%   93.87%   +0.01%     
==========================================
  Files          43       43              
  Lines        9973     9973              
==========================================
+ Hits         9361     9362       +1     
+ Misses        612      611       -1     

Files Changed	Coverage Δ
src/io/matpower.jl	92.92% <100.00%> (ø)

... and 1 file with indirect coverage changes

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