limit PDB output to only include HELIX and SHEET records for included atoms

[Greg Couch]

And example of when there should be no HELIX nor SHEET records:

open 6h67
show :ttd only
write ttd.pdb displayedOnly true

Because when ttd.pdb is opened in ChimeraX, it gives many warnings:

Start residue of secondary structure not found: HELIX 1 1 THR A 20 SER A 28 1 9
Start residue of secondary structure not found: HELIX 2 2 PHE A 90 ALA A 100 1 11
Start residue of secondary structure not found: HELIX 3 3 LYS A 111 LEU A 125 1 15
Start residue of secondary structure not found: HELIX 4 4 LYS A 135 ILE A 139 1 5
Start residue of secondary structure not found: HELIX 5 5 THR A 176 ASP A 199 1 24
293 messages similar to the above omitted