Test multiple neutrals case

[AnjoMan]

Carsons supports models with multiple neutrals. We need to validate that the results are correct. Most of the work involved would be in finding a test case in an existing IEEE model where someone has turned a geometric model into equivalent impedances -- given this, we could easily write a test to ensure that the z_ABC we produce for that cable is correct.

[AnjoMan]

UPDATE: We have some coverage of this now that the library supports concentric neutral cables, which by definition have multiple neutrals.