Add Licence header to existing files if we open-source.

[Jonas-Sander]

See:

https://joinup.ec.europa.eu/sites/default/files/inline-files/EUPL%201_1%20Guidelines%20EN%20Joinup.pdf

4.7 You need to refer to the licence inside the source code.

The choice of the licence must be stated where the software is provided (e.g. on the software's website). There is no need to include the actual text of the licence there, a name and a link to the original licence text will better ensure the reference to a single source. This informs the public what licence the software will be released under, but it's not sufficient for legal purposes. For that, the software code itself must refer to the licence or contain the licence. The standard way to do this is to put the full licence text in a file called “COPYING” (or “LICENCE”), and then put a short notice at the top of each source file, naming the copyright, date, holder and licence, and informing the reader where to find the full text of the licence. An example of “short notice” for software distributed under the EUPL is the following:

/*
* Copyright 2007 YOUR-ORGANISATION4
*
* Licensed under the EUPL, Version 1.1 or – as soon they
will be approved by the European Commission - subsequent
versions of the EUPL (the "Licence");
* You may not use this work except in compliance with the
Licence.
* You may obtain a copy of the Licence at:
*
* https://joinup.ec.europa.eu/software/page/eupl5
*
* Unless required by applicable law or agreed to in
writing, software distributed under the Licence is
distributed on an "AS IS" basis,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
express or implied.
* See the Licence for the specific language governing
permissions and limitations under the Licence.
*/

In the above example, the original author accepts later versions of the EUPL, which could be published by
the European Commission. Distributing “under the EUPL” without any indication of a version number
would produce the same effect, however less explicitly.

Note: The link https://joinup.ec.europa.eu/software/page/eupl5 seems to be outdated. Should be replaced.

[Jonas-Sander]

Closed with #82