Do not repeat the same license in every file

[FichteFoll]

We added the GPL in #42 (issue: #10).

My issue with this is that having the license header on every file is annoying and redundant if it's going to be the same anyway.
We should only need one header for the entire code base, that being LICENSE, and include the whole GPL in a file called LICENSE.GPL3.

Files with a different license (or copyright holder) may then receive their own header as we have it at the moment, but realistically we don't have any file like that right now.

Whether or not we should consider using the LGPL I'll leave up for another issue. That makes more sense to discuss once we get a better picture of how our package should or will be used.