Add SPDX licence and copyright to all source/headers files

[seanm]

It would be nice if Quesa source and header files used SPDX to document their license and copyright.

See: https://spdx.dev/about/overview/

It's very helpful for supply chain and bill of materials tracking.

[jwwalker]

I glanced at the SPDX page, but it wasn't clear to me exactly what it would mean to use SPDX in the headers. I see that GitHub has an option to export an SBOM, but then what?

[seanm]

Having an SBOM is largely the point. It's practically required in regulated fields like medical software.