ReciprocatingWaterToWater and ScrollWaterToWater only allow one stage

[mwetter]

The heat pump models ReciprocatingWaterToWater and ScrollWaterToWater allow for a continuous input signal y and an integer input signal stage.

However, if stage is used, it is internally limited to be between 0 and 1. This is in PartialWaterToWater:

protected 
  Modelica.Blocks.Math.IntegerToReal intToRea
 if enable_variable_speed == false "Conversion for stage signal"
    annotation (Placement(transformation(extent={{-80,-40},{-60,-20}})));

  Modelica.Blocks.Nonlinear.Limiter lim(final uMin=0, final uMax=1)
 if enable_variable_speed == false "Limiter for control signal"
    annotation (Placement(transformation(extent={{-50,-40},{-30,-20}})));

Thus, users can only set stage = 0 or stage = 1, any other value will be set to one of these two. This is consistent with the compressor model which requires an input between 0 and 1.

Also, the model has only one compressor characteristic (as opposed to some DX coils in Buildings that have different performance characteristics for different stages).

Therefore, as stage only allows essentially on or off, I suggest to remove the option of an integer input signal. If users want to model discrete stages, they could do so by feeding only discrete values of y to the model.

@MassimoCimmino : What do you think about this change which is in response to a user inquiry about how to use stages.