Relicense as Apache 2.0

[Bengt]

I would like to combine this project's code with the code from birdnetlib. Since birdnetlib is licensed under the terms of the GPLv3 and this project is license as CC-NC-BY-SA 4.0, that requires licensing this code under the same terms. Please resolve this licensing conflict by relicensing this project under the same license as birdnetlib.

Note that relicensing birdnetlib to CC-NC-BY-SA 4.0 has been discarded, here: https://github.com/joeweiss/birdnetlib/issues/72