Provide multi-chain mapping for UniProt sequences using SIFTs

This will enable mapping multi-chain mapping between a single UniProt sequence and multiple PDB chains using SIFTS.

For instance when lgul_human sequence is fetched from UniProt only a sequence is created. However, when you view the PDB structure for it (1qip), you will discover 4 chains that could be mapped to the same sequence. Current Jalview implementation maps to the structure chain with the maximum match and leaves the rest of the structure chains unmapped.