Added check for positive resistances and reactances

[alexe15]

Thank you so much for this great package! I use it a lot, its my absolute favorite tool to do power system computations.

I've added a check for positive resistances and reactances in make_basic_network. Currently it issues a warning if they are not positive, but maybe it should even be an error. I found cases with negative resistances in the pglib repo, cf. power-grid-lib/pglib-opf#45

[ccoffrin]

Hi @alexe15, I am glad you find power models so useful.

Thank you for the proposal, but I am not inclined to accept this change. Non-positive resistances and reactances are quite comment in real-world network models (as you notice in the PGLib-OPF datasets).

Warning about the existence of such branches is also unrelated to the specific objectives of the "basic data" utilities,
https://lanl-ansi.github.io/PowerModels.jl/stable/basic-data-utilities/