add SPDX information

.gitignore

.gitlab-ci.yml

.mailmap

.reuse/dep5

CHANGELOG.md

CMakeLists.txt

CONTRIBUTING.md

INSTALL

LICENSES/BSD-3-Clause.txt

LICENSES/FSFAP.txt

LICENSES/LicenseRef-GPL-2.0-only-with-DUNE-exception.txt

README.md

TODO

bin/CMakeLists.txt

bin/dune-ctest

bin/dune-git-whitespace-hook

bin/dunecontrol

bin/dunepackaging.py

bin/duneproject

bin/find_modules.py