Using regSquare function in partial valve models to improve robustness

[nieweber]

Currently, the Modelica.Fluid.Utilities.regSquare function is not used in the valve models, but only in the FlowResistance. Adding the regSquare function could improve robustness in edge cases.

The default range, where the regularization starts (delta=0.01) leads to high deviations in the results in edge cases, when the valve is fully closed and the actual mass flow is much smaller than the reference mass flow. Future tests should investigate, how the robustness can be improved using the regSquare functions with different regularization ranges delta.