Use REUSE and SPDX to declare copyright and license information

[nbarrientos]

[~/dev/jens]@malt
λ reuse lint
# SUMMARY

* Bad licenses:
* Deprecated licenses:
* Licenses without file extension:
* Missing licenses:
* Unused licenses:
* Used licenses: CC0-1.0, GPL-3.0-or-later
* Read errors: 0
* Files with copyright information: 77 / 77
* Files with license information: 77 / 77

Congratulations! Your project is compliant with version 3.0 of the REUSE Specification :-)

The patch also adds a CI job to ensure compliance.

[nbarrientos]

Please @giacomd ping me back when the CERN statement bit is clarified so I can dispatch this change request for review. Thanks.