Make mindeltaT a parameter in AixLib.Fluid.DistrictHeatingCooling.Demands.OpenLoop.VarTSupplyDp

[j-zipplies]

Describe the bug
This applies to the open loop demand models with a fixed return temperature:
AixLib.Fluid.DistrictHeatingCooling.Demands.OpenLoop.VarTSupplyDp
AixLib.Fluid.DistrictHeatingCooling.Demands.OpenLoop.VarTSupplyDpBypass
From the documentation and parameters of the model, the user will expect that the demand model has a fixed return temperature.
However, it also has a minimum temperature difference, which is set via the block "mindeltaT", with a value of "k=20", which overrides the setpoint TReturn, whenever this mindeltaT would be fallen short of. This minimum temperature difference of 20 K is quite high and it is hidden from the users, as it is hard coded and not a parameter.

Expected behavior
I strongly encourage:

• to add a parameter "dT_min" to the model with a default of 20 K (or below) and then set mindeltaT(k=dT_min).
• add a hint to this feature to the documentation