Fix copyright starting dates

[lukaspie]

In some files, the starting date for the copyright clause have been changed (e.g. NXroot). We should ensure in the future that

1. we don't change existing dates
2. we reset the changed dates
3. nyaml2nxdl picks up the correct starting date in agreement with what NIAC is proposing.

2. The starting dates are already changed in our repository for some files. I don't know the latest status of the copyright discussion.

Originally posted by @domna in #137 (comment)

[RubelMozumder]

The issue of NYAML licensing (FAIRmat-NFDI/nyaml#44) will be handled here.

[lukaspie]

We should not only change nyaml so that it does not overwrite the copyright starting dates, but also check which files in the fairmat branch have a copyright date that is different from the original one in the NIAC repo.