This model outputs the relative humidity of the fluid flowing from
port_a to port_b.
The sensor is ideal, i.e., it does not influence the fluid.
Note that this sensor can only be used with media that contain the variable phi,
which is typically the case for moist air models.
If the parameter tau is non-zero, then its output
is computed using a first order differential equation.
Setting tau=0 is not recommend. See
Buildings.Fluid.Sensors.UsersGuide for an explanation.
quantity attribute for mass fraction variables.tauInv
since this now exists in
Buildings.Fluid.Sensors.BaseClasses.PartialDynamicFlowSensor.
This is for
#372.