Change license to BSD-3
With https://github.com/ibpsa/modelica-ibpsa/pull/897, the Modelica IBPSA Library changed from Modelica 2 license to BSD-3 clause license, as did the MSL with https://github.com/modelica/ModelicaStandardLibrary/pull/2373. Since the Modelica 2 license do actually not allow integrating a library into another, we should also switch to another license. I would prefer MIT, but since all others use BSD-3, let's do the same. More information about the licenses can be found here https://choosealicense.com/licenses/ and here https://de.wikipedia.org/wiki/BSD-Lizenz. Any comments or preferences?