SIFTS map uniprot resnum to PDB function fix

[JoshuaMeyers]

This MR fixes the function pdb.map_uniprot_resnum_to_pdb. Previously it read chains with the assumption that chain A == entity A. If chains IDs in the PDB start later in the alphabet, this caused issues (i.e. PDB ID: 1atp chain I)

Now the code reads unique chains in the SIFTs file. And then loops through them alphabetically. so that the first chain -> A, second -> B. etc.

This is a precursor to my open issue (https://github.com/SBRG/ssbio/issues/51). I hope its useful.

The tests pass, but I have not fixed the existing broken test in the same file (test_download_biomol)