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

docs/protobuf/README.md

@@ -1,3 +1,7 @@
+{::comment}
+SPDX-License-Identifier: GPL-3.0-or-later
+SPDX-FileCopyrightText: 2018-2020 Carles Fernandez-Prades <carles.fernandez@cttc.es>
+{:/comment}
 # Custom structured data format definitions
 
 Files in this folder describe structured data formats that are generated by

docs/protobuf/gnss_synchro.proto

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: BSD-3-Clause
+// SPDX-FileCopyrightText: 2018-2020 Carles Fernandez-Prades <carles.fernandez@cttc.es>
 syntax = "proto3";
 
 package gnss_sdr;

docs/protobuf/monitor_pvt.proto

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: BSD-3-Clause
+// SPDX-FileCopyrightText: 2018-2020 Carles Fernandez-Prades <carles.fernandez@cttc.es>
 syntax = "proto3";
 
 package gnss_sdr;