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

.clang-tidy

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+# SPDX-FileCopyrightText: 2018-2020 Carles Fernandez-Prades <carles.fernandez@cttc.es>
 ---
 Checks: '-*,
         boost-use-to-string,