Define a β sheet.
strand: Strand number which starts at 1 for each strand within a sheet and increases by one.
sheet_id: The sheet identifier. To match the PDB standard, sheet IDs should range from `A' to `Z'.
strand_count: The number of strands in the sheet.
strand_sense: Sense of strand with respect to previous strand in the sheet. 0 if first strand, 1 if parallel, -1 if anti-parallel.
start: The residue number for the start of the sheet.
end: The residue number for the end of the sheet.
mol_name: Define the secondary structure for a specific molecule.
current_atom: The name of the first atom in the current strand, to link the current back to the previous strand.
prev_atom: The name of the last atom in the previous strand, to link the current back to the previous strand.
This allows β sheet secondary structure to be defined for the internal structural object.