Chains missing without DSSP

[BradyAJohnston]

When not using the DSSP node or tickbox for Style Cartoon, we lose a bunch of chains in some structures.

Example structure: 9G6V

[x] DSSP	[ ] DSSP

It seems that all of the other chains are being assigned sec_struct = 0 when importing, which is why they are being dropped.

[BradyAJohnston]

It is also displayed without secondary structure on the other chains on the PDB, so it's not a parsing issue but the data just seems to be stored that way for whatever reason (could still be handled better)