Move to GNU General Public License v3.0 licence

[lucduron]

Hi @tomsail,

I noticed that the current licence of xarray-selafin is "The Unlicense" (see file LICENSE at root).
What do you think of switching to a "GNU General Public License v3.0 licence"?
Indeed xarray-selafin relies on PyTelTools (a copied subset) which has this type of licence.
I suggest to replace LICENCE par LICENCE.txt (copy content from https://github.com/CNR-Engineering/PyTelTools/blob/master/LICENSE.txt).

Best Regards,
Luc

[tomsail]

Sure, as long we remain fully open source no problem.
Most of our codes are GPL3 too, I don't remember why I chose Unilicense in the first time.. maybe to be more permissive.

[tomsail]

fixed with 57c8972