Allow EESM parameters that are already transformed to stator side

[max-schenke]

In the current configuration, GEM expects EESM parameters to be the physical quantities of either the rotor or the stator side. The rotor-side parameters are then by default transformed to the stator side for further calculations.

For convenience reasons, it should be allowed to specify rotor parameters that are already transformed to the stator side. The variables have different names and the transformation would need to be skipped in that case (or reverted to yield the physical circuit quantities, which are not needed for calculation but could then be retrieved from the environment).