Simplify moisture fraction when allowFlowReversal=false

[Mathadon]

Annex60.Fluid.Interfaces.PartialTwoPortTransport can be simplified when allowFlowReversal=false:

  port_a.Xi_outflow =  inStream(port_b.Xi_outflow);
  port_b.Xi_outflow = inStream(port_a.Xi_outflow);
  port_a.C_outflow = inStream(port_b.C_outflow);
  port_b.C_outflow = inStream(port_a.C_outflow);

then becomes

  port_a.Xi_outflow = if allowFlowReversal then inStream(port_b.Xi_outflow) else Medium.X_default[1:Medium.nXi];
  port_b.Xi_outflow = inStream(port_a.Xi_outflow);
  port_a.C_outflow = if allowFlowReversal then inStream(port_b.C_outflow) else zeros(Medium.nC);
  port_b.C_outflow = inStream(port_a.C_outflow);

this is similar to Annex60.Fluid.Interfaces.StaticTwoPortConservationEquation.

I'll make a pull request.

[mwetter]

This is now on the master.