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.