Adapt new Modelica Association license (BSD-3 clause)

[beutlich]

The library is still Licensed by the Modelica Association under the Modelica License 2. Since the library is within the modelica user group and therefore under the umbrella of the Modelica Association, the new licensing model of the Modelica Association should be adapted (i.e., BSD-3 clause). This would help for future integration into the Modelica Standard Library (MSL).

@christoff-buerger I copied the issue text from modelica-deprecated/Modelica_Synchronous#35.