Enable computeFlowResistance=false

[FWuellhorst]

To simplify complex hydraulic models, it should be possible to disable the pressure loss equation in a model but still use the dp_nominal and aggregate the value with other dp_nominals in series to one dpFixed_nominal of an actuator.
As discussed and proposed by @Mathadon , the parameter

protected 
  final parameter Boolean computeFlowResistance=(dp_nominal_pos > Modelica.Constants.eps)
    "Flag to enable/disable computation of flow resistance"
   annotation(Evaluate=true);

in IBPSA.Fluid.FixedResistances.PressureDrop could be public and non-final in a tab advanced.