Adopt SPDX Licence Identifiers and REUSE-Compliant File Headers

[aaron-hopkinson]

We need to change the license headers across the anemoi packages to follow the recently approved ECMWF ADR, see: https://github.com/ecmwf/codex/blob/main/ADR/ADR-010-SPDX-License-Identifiers-and-REUSE-Headers.md