Make the software package compliant with the REUSE Specification v3.0 (see https://reuse.software/spec/)

Update license headers to SPDX format (see https://spdx.org/)
Add license to all files
Add CI job in GitHub Actions to ensure compliance

CODE_OF_CONDUCT.md

@@ -1,3 +1,7 @@
+{::comment}
+SPDX-License-Identifier: GPL-3.0-or-later
+SPDX-FileCopyrightText: 2011-2020 Carles Fernandez-Prades <carles.fernandez@cttc.es>
+{:/comment}
 # Contributor Covenant Code of Conduct
 
 ## Our Pledge