small bug in atom3d.util.formats.df_to_bps line 111

[YangyangMiao]

Hi,
I found a small bug that will make the biopython structure output have error when it do io.save .
if you check the chain.id write into . it will be a tuple instead of a char.
The modification will be
new_chain = Bio.PDB.Chain.Chain(chain) -> new_chain = Bio.PDB.Chain.Chain(chain[0])

Best
Yangyang