docs: def pdb(self):
          """Compiles the PDB strings for each state into a single file."""
          header_title = '{:<80}\n'.format('HEADER    {}'.format(self.id))
          data_type = '{:<80}\n'.format('EXPDTA    ISAMBARD Model')
          pdb_strs = []
          for ampal in self:
              if isinstance(ampal, Assembly):
                  pdb_str = ampal.make_pdb(header=False, footer=False)
              else:
                  pdb_str = ampal.make_pdb()
              pdb_strs.append(pdb_str)
          merged_strs = 'ENDMDL\n'.join(pdb_strs) + 'ENDMDL\n'
          merged_pdb = ''.join([header_title, data_type, merged_strs])
          return merged_pdb