Support multiple secondary structures per fragment #260
Description
Split from #259:
In our current system representation, fragments can be part of at most one secondary structure. However, PDB structures such as 1GJC and 8QJB declare certain fragments to be part of multiple beta sheets, either as a different strand of the same sheet or as a strand of a separate sheet:
1GJC
SHEET 1 A 7 PHE B 30 ARG B 36 0 <---
SHEET 2 A 7 VAL B 38 SER B 48 -1 O THR B 39 N ARG B 35
SHEET 3 A 7 TRP B 51 SER B 54 -1 O TRP B 51 N MET B 47
SHEET 4 A 7 ALA B 104 ARG B 109 -1 O ALA B 104 N SER B 54
SHEET 5 A 7 MET B 81 ILE B 89 -1 N GLU B 84 O ARG B 109
SHEET 6 A 7 TYR B 64 LEU B 68 -1 O TYR B 64 N VAL B 85
SHEET 7 A 7 PHE B 30 ARG B 36 -1 N ALA B 32 O TYR B 67 <--- same sheet, different strand
8QJB
SHEET 1 AA3 8 ASP A 428 ARG A 432 0 <---
SHEET 2 AA3 8 ASP A 418 THR A 422 -1 N PHE A 421 O LEU A 429
SHEET 3 AA3 8 MET A 411 HIS A 415 -1 N HIS A 415 O ASP A 418
SHEET 4 AA3 8 ILE A 480 HIS A 487 -1 O TYR A 481 N LEU A 412
SHEET 5 AA3 8 HIS A 469 LEU A 475 -1 N ILE A 472 O ALA A 485
SHEET 6 AA3 8 LEU A 460 ASP A 464 -1 N ILE A 462 O GLN A 471
SHEET 7 AA3 8 GLN A 447 ASP A 455 -1 N ARG A 451 O PHE A 463
SHEET 8 AA3 8 ILE A 438 ASN A 444 -1 N PHE A 439 O GLY A 454
SHEET 1 AA479 ASP A 428 ARG A 432 0 <-- different sheet