Issue205 transformer loss

I added my new transformer model in the requested branch. I created this model earlier this year for a benchmark. This model is equal to the model used in the Matlab-code (three-phase unbalanced power flow) by Jeroen Tant/Frederik Geth (Electa). As discussed via mail this morning, this should reduce the total loses (and therefore the ratio PlosTot/P0).

Note that you should still adapt the transformer model in the three-phase grid model in order to use it, since I left the old model in the library.
The model should work, as it is a copy/paste from my phd-library (incl. the required adaptations with the structure of the latest IDEAS library).