mapping pdb resnum to uniprot resnum

[JoshuaMeyers]

Hey nice library. I am interested in this function for reading the SIFTS residue XML files (which are a pain to parse!)

ssbio.databases.pdb.map_uniprot_resnum_to_pdb

Do you have any tips for going the other direction (I.e. map_pdb_resnum_to_uniprot)? Otherwise could this be a useful addition?