Fix a remark in the type model of a two winding transformer

[dzanansarajlic]

For the attribute xSc it says in the remarks that it is the short circuit reactance, but this should be the short circuit impedance.

[johanneshiry]

Please also check javadocs accordingly when addressing this

[staudtMarius]

@t-ober @danielfeismann
StandardUnits split impedance_per_length and impedance into standard units for reactance and resistance.