DSSP: Fix ValueError: too many values to unpack in __main__

Commit 3e287df caused a minor issue since __iter__ and keys methods of DSSP class return different tuples. DSSP objects cannot be indexed by tuples returned by __iter__. 

Either [d[x][1] for x in d.keys()] or [x[1] for x in d] must be used to get the secondary structure.