Map PDB sequential and author numbering

[AntonPetrov]

For example, the new Rfam 3D seed RF0XXXX for PDB XXXX includes a sequence ACGU...CGG, but what are the nucleotide numbers of these nucleotides in the PDB file? That numbering does not have to be sequential, it can start from 5 and not from 1 for example.

The mapping would need to be provided in separate files, possibly following the SIFTS conventions developed for proteins.

This feature was requested by Yann at the 2022 SAB meeting.