Do a package for converting DL/UL NR-ARFCN in between each other and to
convert DL NR-ARFCN to SSB NR-ARFCN. The API mimics xlte.earfcn added in 6cb9d37f.

xlte.nrarfcn complements pypi.org/project/nrarfcn, which we use here under the hood.

See package documentation for API details.