Modelica.Fluid.Sensors.BaseClasses.PartialFlowSensor should have simplified equations if allowFlowReversal = false

[casella]

Modelica.Fluid.Sensors.BaseClasses.PartialFlowSensor currently contains these equations:

https://github.com/modelica/ModelicaStandardLibrary/blob/ea28ae3e29bf0ed9c68189100292a0001151143e/Modelica/Fluid/Sensors.mo#L828-L835

In fact, it makes no sense to compute the inStream() quantities at port_a if allowFlowReversal = false, because they will never be used. In that case, we should give them some constant default value.