Make kFixed final in PartialTwoWayValve and CheckValve

[AntoineGautier]

In IBPSA.Fluid.Actuators.BaseClasses.PartialTwoWayValve and IBPSA.Fluid.FixedResistances.CheckValve it seems that the following declaration should be made final.

  parameter Real kFixed(unit="", min=0) = if dpFixed_nominal > Modelica.Constants.eps
    then m_flow_nominal / sqrt(dpFixed_nominal) else 0
    "Flow coefficient of fixed resistance that may be in series with valve, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2).";

I do not any see any use case where the user should be allowed to modify that assignment.
Also, the class IBPSA.Fluid.Actuators.BaseClasses.PartialDamperExponential uses the following.

  final parameter Real kFixed = if dpFixed_nominal > Modelica.Constants.eps then 
    m_flow_nominal / sqrt(dpFixed_nominal) else Modelica.Constants.inf
    "Flow coefficient of fixed resistance that may be in series with damper, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2).";