Checks to make sure that each atom type is unique per residue.

def check_residues(self):
        '''Checks to make sure that each atom type is unique per residue.'''
        for chain, residue_ids in self.residues.iteritems():
            for residue_id, residue in residue_ids.iteritems():
                for record_type, atoms in residue.records.iteritems():
                    freq = {}
                    for atom in atoms:
                        rec_id = atom.name + atom.conformation
                        freq[rec_id] = freq.get(rec_id, 0)
                        freq[rec_id] += 1
                    for atom_type, count in freq.items():
                        if count > 1:
                            raise Exception('{0} occurrences of atom type {1} for record type {2} occur in residue {3} in chain {4}.'.format(count, atom_type, record_type, residue_id.strip(), chain))