PowerModels v0.18.3

[hhijazi]

With version v0.18.3 of PowerModels, one line needs to be changed in modelopf.jl: ref = PowerModels.build_ref(data)[:nw][0] should become: ref = PowerModels.build_ref(data)[:it][pm_it_sym][:nw][0] Otherwise, one gets the following error: LoadError: KeyError: key :nw not found