[extract module] add option to include GPLv3 license to new module

What this MR does / why does DuMux need it:

Fixes #1202 (closed) . This MR adds the option to automatically add a GPLv3 license file to a new module when executing the extract_as_new_module.py script as discussed in issue #1202 (closed) . When choosing to add a license, licensing information is also added to the README.md file. To my understanding, this is necessary, because in the past, without this option, it would happen regularly that new modules would be extracted and published without proper licensing, which is critical. This especially concerns code accompanying scientific papers that are published in the dumux-pub repository.

Notes for the reviewer

I put the license file in a directory in the bin directory, because I think that adding such a long text directly into the code will make the unclear and hard to read and understand. If there are better alternatives to store the license, this could be changed easily, in my opinion.

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