Add Modelica License 2

[AnHeuermann]

The Modelica License 2 is very hard to find (I only found it in a web archive of https://modelica.org/licenses/ModelicaLicense2/).

Include the license in the repository and in the top level of PNlib:

How to Apply the Modelica License 2

At the top level of your Modelica package and at every important subpackage, add the following notices in the info layer of the package:

Licensed by <Licensor> under the Modelica License 2
Copyright © <year1>-<year2>, <name of copyright holder(s)>.

This Modelica package is free software and the use is completely at your own risk; it can be redistributed and/or modified under the terms of the Modelica License 2. For license conditions (including the disclaimer of warranty) see Modelica.UsersGuide.ModelicaLicense2 or visit http://www.modelica.org/licenses/ModelicaLicense2.

Include a copy of the Modelica License 2 under <library>.UsersGuide.ModelicaLicense2 (use http://www.modelica.org/licenses/ModelicaLicense2.mo). Furthermore, add the list of authors and contributors under <library>.UsersGuide.Contributors or <library>.UsersGuide.Contact.

For example, sublibrary Modelica.Blocks of the Modelica Standard Library may have the following notices:

Licensed by Modelica Association under the Modelica License 2
Copyright © 1998-2008, Modelica Association.

This Modelica package is free software and the use is completely at your own risk; it can be redistributed and/or modified under the terms of the Modelica License 2. For license conditions (including the disclaimer of warranty) see Modelica.UsersGuide.ModelicaLicense2 or visit http://www.modelica.org/licenses/ModelicaLicense2.

For C-source code and documents, add similar notices in the corresponding file.

For images, add a "readme.txt" file to the directories where the images are stored and include a similar notice in this file.

In these cases, save a copy of the Modelica License 2 in one directory of the distribution, e.g., http://www.modelica.org/licenses/ModelicaLicense2.html in directory <library>/Resources/Documentation/ModelicaLicense2.html.

[AnHeuermann]

A license that relies on being always online available (and isn't any more) is probably not a good idea.
Maybe a different license should be considered.