Re-add license statement [Revert "Rename COPYING to LICENSE"]

GPAW has for some years had a LICENSE file which contains a license text, but apparently no statement of the type "GPAW is released under this license".

This MR reestablishes the previous licensing statement which AFAIK was correct: LICENSE states the license (which is the important thing) whereas COPYING contains the license text (which is probably unimportant. But it's recommended to include it).

If we do not have a license statement, then it is not clearly stated anywhere how that license text relates to the code or, if one assumes that it applies to the code, whether it's GPLv3 or GPLv3+.

This reverts commit 40591b61.