Make TwoWindingTransformer use the new p and n pin

Prior to this commit, positive and negative pins had the same appearence.
Their meaning is however dfifferent since their voltages and currents are different.
Now the positive pin will be blue-fitted, and the negative pin white fitted in compliance with that is done throughout the Modelica Standard Library.

Author: ceraolo

OpenIPSL/Electrical/Branches/PSAT/TwoWindingTransformer.mo

@@ -5,9 +5,9 @@ model TwoWindingTransformer "Modeled as series reactances without iron losses"
   import Modelica.ComplexMath.real;
   import Modelica.ComplexMath.imag;
   import Modelica.ComplexMath.j;
-  OpenIPSL.Interfaces.PwPin p
+  OpenIPSL.Interfaces.PwPin_p p
     annotation (Placement(transformation(extent={{-120,-10},{-100,10}})));
-  OpenIPSL.Interfaces.PwPin n
+  OpenIPSL.Interfaces.PwPin_n n
     annotation (Placement(transformation(extent={{100,-10},{120,10}})));
   parameter Types.ApparentPower S_b=SysData.S_b "System base power"
     annotation (Dialog(group="Power flow"));