relicensing LLGPL → GPLv3

[zzkt]

Based on the discussion and analysis on cliki about the LLGPL (in particular Eli Greenbaum's detailed examination of the LLGPL) I think it makes sense to change the licence to GPLv3 (or later).

If there is anyone who has contributed to the library that objects to this please let me know.

[zzkt]

cc @PuercoPop @boqs @erikronstrom @jamieforth

[PuercoPop]

@zzkt I'm always in favor of the strongest copyleft possible until the time IP is abolished. If anything I'd recommend AGPLv3! 👍