Compiles the PDB strings for each state into a single file.

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