Restructure/Normalize Transformer Models

[stephen-frank]

The transformer model currently accepts all parameters in volt-ampere units. This makes scaling the model quite difficult when voltage, current, and power ratings change. The transformer model should be modified to use per-unit parameters together with ratings, such that the transformer model can be modified easily by solely changing the device ratings (leaving all the model parameters, in per-unit, the same).

This also has the advantage that per-unit values for transformer parameters can be easily evaluated at a glance for conformance with expected ranges. (E.G. series reactance in the range 0.02-0.10).