Make the repo REUSE compliant and add a CI check

[mattkretz]

ChangeLog:

* .github/workflows/CI.yml:
* .github/workflows/fair-software.yml:
* .github/workflows/reuse.yml: New file.
* .gitignore:
* .pre-commit-config.yaml: New file.
* CITATION.cff:
* CMakeLists.txt:
* CODE_OF_CONDUCT.md.license: New file.
* CONTRIBUTING.md.license: New file.
* LICENSES/Apache-2.0.txt: New file.
* LICENSES/CC0-1.0.txt: New file.
* LICENSES/GPL-3.0-or-later.txt: New file.
* LICENSES/LGPL-3.0-or-later.txt: New file.
* Makefile:
* README.md.license: New file.
* latest_std_flag.sh:
* testsuite/README.md.license: New file.
* testsuite/driver.sh:
* testsuite/generate_makefile.sh:
* testsuite/standard_abi_usable_2.cc:
* testsuite/tests/bitset.cc:
* testsuite/tests/for_each.cc:
* testsuite/tests/transform.cc:
* testsuite/tests/transform_reduce.cc: